Bug 606145 part 2 - Import elfhack code. r=tglek,a=bsmedberg

build/unix/elfhack/README

@@ -0,0 +1,28 @@
+Elfhack is a program to optimize ELF binaries for size and cold startup
+speed.
+
+Presently, it is quite experimental, though it works well for the target
+it was created for: Firefox's libxul.so.
+
+Elfhack currently only does one thing: packing dynamic relocations ;
+which ends up being a quite complex task, that can be summarized this
+way:
+- Remove RELATIVE relocations from the .rel.dyn/.rela.dyn section.
+- Inject a small code able to apply relative relocations "by hand"
+  after the .rel.dyn/.rela.dyn section.
+- Inject a section containing relocative relocations in a different
+  and more packed format, after the small code.
+- Register the small code as DT_INIT function. Make the small code call
+  what was initially the DT_INIT function, if there was one.
+- Remove the hole between the new section containing relative
+  relocations and the following sections, adjusting offsets and base
+  addresses accordingly.
+- Adjust PT_LOAD entries to fit new offsets, and add an additional
+  PT_LOAD entry when that is necessary to handle the discrepancy between
+  offsets and base addresses, meaning the section offsets may yet again
+  need adjustments.
+- Adjust various DT_* dynamic tags to fit the new ELF layout.
+- Adjust section headers.
+- Adjust ELF headers.
+
+See http://glandium.org/blog/?p=1177#relocations for some figures.

build/unix/elfhack/dummy.c

@@ -0,0 +1,3 @@
+int main() {
+    return 1;
+}

build/unix/elfhack/elf.cpp

@@ -0,0 +1,822 @@
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is elfhack.
+ *
+ * The Initial Developer of the Original Code is
+ * Mozilla Foundation.
+ * Portions created by the Initial Developer are Copyright (C) 2010
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   Mike Hommey <mh@glandium.org>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+#include <cstring>
+#include <assert.h>
+#include "elfxx.h"
+
+template <class endian, typename R, typename T>
+inline void Elf_Ehdr_Traits::swap(T &t, R &r)
+{
+    memcpy(r.e_ident, t.e_ident, sizeof(r.e_ident));
+    r.e_type = endian::swap(t.e_type);
+    r.e_machine = endian::swap(t.e_machine);
+    r.e_version = endian::swap(t.e_version);
+    r.e_entry = endian::swap(t.e_entry);
+    r.e_phoff = endian::swap(t.e_phoff);
+    r.e_shoff = endian::swap(t.e_shoff);
+    r.e_flags = endian::swap(t.e_flags);
+    r.e_ehsize = endian::swap(t.e_ehsize);
+    r.e_phentsize = endian::swap(t.e_phentsize);
+    r.e_phnum = endian::swap(t.e_phnum);
+    r.e_shentsize = endian::swap(t.e_shentsize);
+    r.e_shnum = endian::swap(t.e_shnum);
+    r.e_shstrndx = endian::swap(t.e_shstrndx);
+}
+
+template <class endian, typename R, typename T>
+inline void Elf_Phdr_Traits::swap(T &t, R &r)
+{
+    r.p_type = endian::swap(t.p_type);
+    r.p_offset = endian::swap(t.p_offset);
+    r.p_vaddr = endian::swap(t.p_vaddr);
+    r.p_paddr = endian::swap(t.p_paddr);
+    r.p_filesz = endian::swap(t.p_filesz);
+    r.p_memsz = endian::swap(t.p_memsz);
+    r.p_flags = endian::swap(t.p_flags);
+    r.p_align = endian::swap(t.p_align);
+}
+
+template <class endian, typename R, typename T>
+inline void Elf_Shdr_Traits::swap(T &t, R &r)
+{
+    r.sh_name = endian::swap(t.sh_name);
+    r.sh_type = endian::swap(t.sh_type);
+    r.sh_flags = endian::swap(t.sh_flags);
+    r.sh_addr = endian::swap(t.sh_addr);
+    r.sh_offset = endian::swap(t.sh_offset);
+    r.sh_size = endian::swap(t.sh_size);
+    r.sh_link = endian::swap(t.sh_link);
+    r.sh_info = endian::swap(t.sh_info);
+    r.sh_addralign = endian::swap(t.sh_addralign);
+    r.sh_entsize = endian::swap(t.sh_entsize);
+}
+
+template <class endian, typename R, typename T>
+inline void Elf_Dyn_Traits::swap(T &t, R &r)
+{
+    r.d_tag = endian::swap(t.d_tag);
+    r.d_un.d_val = endian::swap(t.d_un.d_val);
+}
+
+template <class endian, typename R, typename T>
+inline void Elf_Sym_Traits::swap(T &t, R &r)
+{
+    r.st_name = endian::swap(t.st_name);
+    r.st_value = endian::swap(t.st_value);
+    r.st_size = endian::swap(t.st_size);
+    r.st_info = t.st_info;
+    r.st_other = t.st_other;
+    r.st_shndx = endian::swap(t.st_shndx);
+}
+
+template <class endian>
+struct _Rel_info {
+    static inline void swap(Elf32_Word &t, Elf32_Word &r) { r = endian::swap(t); }
+    static inline void swap(Elf64_Xword &t, Elf64_Xword &r) { r = endian::swap(t); }
+    static inline void swap(Elf64_Xword &t, Elf32_Word &r) {
+        r = endian::swap(ELF32_R_INFO(ELF64_R_SYM(t), ELF64_R_TYPE(t)));
+    }
+    static inline void swap(Elf32_Word &t, Elf64_Xword &r) {
+        r = endian::swap(ELF64_R_INFO(ELF32_R_SYM(t), ELF32_R_TYPE(t)));
+    }
+};
+
+template <class endian, typename R, typename T>
+inline void Elf_Rel_Traits::swap(T &t, R &r)
+{
+    r.r_offset = endian::swap(t.r_offset);
+    _Rel_info<endian>::swap(t.r_info, r.r_info);
+}
+
+template <class endian, typename R, typename T>
+inline void Elf_Rela_Traits::swap(T &t, R &r)
+{
+    r.r_offset = endian::swap(t.r_offset);
+    _Rel_info<endian>::swap(t.r_info, r.r_info);
+    r.r_addend = endian::swap(t.r_addend);
+}
+
+static const Elf32_Shdr null32_section =
+    { 0, SHT_NULL, 0, 0, 0, 0, SHN_UNDEF, 0, 0, 0 };
+
+Elf_Shdr null_section(null32_section);
+
+Elf_Ehdr::Elf_Ehdr(std::ifstream &file, char ei_class, char ei_data)
+: serializable<Elf_Ehdr_Traits>(file, ei_class, ei_data),
+  ElfSection(null_section, NULL, NULL)
+{
+    shdr.sh_size = Elf_Ehdr::size(ei_class);
+}
+
+Elf::Elf(std::ifstream &file)
+{
+    if (!file.is_open())
+        throw std::runtime_error("Error opening file");
+
+    file.exceptions(std::ifstream::eofbit | std::ifstream::failbit | std::ifstream::badbit);
+    // Read ELF magic number and identification information
+    char e_ident[EI_VERSION];
+    file.seekg(0);
+    file.read(e_ident, sizeof(e_ident));
+    file.seekg(0);
+    ehdr = new Elf_Ehdr(file, e_ident[EI_CLASS], e_ident[EI_DATA]);
+
+    // ELFOSABI_LINUX is kept unsupported because I haven't looked whether
+    // STB_GNU_UNIQUE or STT_GNU_IFUNC would need special casing.
+    if ((ehdr->e_ident[EI_OSABI] != ELFOSABI_NONE) && (ehdr->e_ident[EI_ABIVERSION] != 0))
+        throw std::runtime_error("unsupported ELF ABI");
+
+    if (ehdr->e_version != 1)
+        throw std::runtime_error("unsupported ELF version");
+
+    // Sanity checks
+    if (ehdr->e_shnum == 0)
+        throw std::runtime_error("sstripped ELF files aren't supported");
+
+    if (ehdr->e_ehsize != Elf_Ehdr::size(e_ident[EI_CLASS]))
+        throw std::runtime_error("unsupported ELF inconsistency: ehdr.e_ehsize != sizeof(ehdr)");
+
+    if (ehdr->e_shentsize != Elf_Shdr::size(e_ident[EI_CLASS]))
+        throw std::runtime_error("unsupported ELF inconsistency: ehdr.e_shentsize != sizeof(shdr)");
+
+    if (ehdr->e_phnum == 0) {
+        if (ehdr->e_phoff != 0)
+            throw std::runtime_error("unsupported ELF inconsistency: e_phnum == 0 && e_phoff != 0");
+        if (ehdr->e_phentsize != 0)
+            throw std::runtime_error("unsupported ELF inconsistency: e_phnum == 0 && e_phentsize != 0");
+    } else if (ehdr->e_phoff != ehdr->e_ehsize)
+        throw std::runtime_error("unsupported ELF inconsistency: ehdr->e_phoff != ehdr->e_ehsize");
+    else if (ehdr->e_phentsize != Elf_Phdr::size(e_ident[EI_CLASS]))
+        throw std::runtime_error("unsupported ELF inconsistency: ehdr->e_phentsize != sizeof(phdr)");
+
+    // Read section headers
+    Elf_Shdr **shdr = new Elf_Shdr *[ehdr->e_shnum];
+    file.seekg(ehdr->e_shoff);
+    for (int i = 0; i < ehdr->e_shnum; i++)
+        shdr[i] = new Elf_Shdr(file, e_ident[EI_CLASS], e_ident[EI_DATA]);
+
+    // Sanity check in section header for index 0
+    if ((shdr[0]->sh_name != 0) || (shdr[0]->sh_type != SHT_NULL) ||
+        (shdr[0]->sh_flags != 0) || (shdr[0]->sh_addr != 0) ||
+        (shdr[0]->sh_offset != 0) || (shdr[0]->sh_size != 0) ||
+        (shdr[0]->sh_link != SHN_UNDEF) || (shdr[0]->sh_info != 0) ||
+        (shdr[0]->sh_addralign != 0) || (shdr[0]->sh_entsize != 0))
+        throw std::runtime_error("Section header for index 0 contains unsupported values");
+
+    if ((shdr[ehdr->e_shstrndx]->sh_link != 0) || (shdr[ehdr->e_shstrndx]->sh_info != 0))
+        throw std::runtime_error("unsupported ELF content: string table with sh_link != 0 || sh_info != 0");
+
+    // Store these temporarily
+    tmp_shdr = shdr;
+    tmp_file = &file;
+
+    // Fill sections list
+    sections = new ElfSection *[ehdr->e_shnum];
+    for (int i = 0; i < ehdr->e_shnum; i++)
+        sections[i] = NULL;
+    for (int i = 1; i < ehdr->e_shnum; i++) {
+        if (sections[i] != NULL)
+            continue;
+        getSection(i);
+    }
+    Elf_Shdr s;
+    s.sh_name = 0;
+    s.sh_type = SHT_NULL;
+    s.sh_flags = 0;
+    s.sh_addr = 0;
+    s.sh_offset = ehdr->e_shoff;
+    s.sh_entsize = Elf_Shdr::size(e_ident[EI_CLASS]);
+    s.sh_size = s.sh_entsize * ehdr->e_shnum;
+    s.sh_link = 0;
+    s.sh_info = 0;
+    s.sh_addralign = (e_ident[EI_CLASS] == ELFCLASS32) ? 4 : 8;
+    shdr_section = new ElfSection(s, NULL, NULL);
+
+    // Fake section for program headers
+    s.sh_offset = ehdr->e_phoff;
+    s.sh_entsize = Elf_Phdr::size(e_ident[EI_CLASS]);
+    s.sh_size = s.sh_entsize * ehdr->e_phnum;
+    phdr_section = new ElfSection(s, NULL, NULL);
+
+    phdr_section->insertAfter(ehdr);
+
+    sections[1]->insertAfter(phdr_section);
+    for (int i = 2; i < ehdr->e_shnum; i++) {
+        // TODO: this should be done in a better way
+        if ((shdr_section->getPrevious() == NULL) && (shdr[i]->sh_offset > ehdr->e_shoff)) {
+            shdr_section->insertAfter(sections[i - 1]);
+            sections[i]->insertAfter(shdr_section);
+        } else
+            sections[i]->insertAfter(sections[i - 1]);
+    }
+    if (shdr_section->getPrevious() == NULL)
+        shdr_section->insertAfter(sections[ehdr->e_shnum - 1]);
+
+    tmp_file = NULL;
+    tmp_shdr = NULL;
+    for (int i = 0; i < ehdr->e_shnum; i++)
+        delete shdr[i];
+    delete[] shdr;
+
+    eh_shstrndx = (ElfStrtab_Section *)sections[ehdr->e_shstrndx];
+
+    // Skip reading program headers if there aren't any
+    if (ehdr->e_phnum == 0)
+        return;
+
+    // Read program headers
+    file.seekg(ehdr->e_phoff);
+    for (int i = 0; i < ehdr->e_phnum; i++) {
+        Elf_Phdr phdr(file, e_ident[EI_CLASS], e_ident[EI_DATA]);
+        ElfSegment *segment = new ElfSegment(&phdr);
+        // Some segments aren't entirely filled (if at all) by sections
+        // For those, we use fake sections
+        if ((phdr.p_type == PT_LOAD) && (phdr.p_offset == 0)) {
+            // Use a fake section for ehdr and phdr
+            ehdr->getShdr().sh_addr = phdr.p_vaddr;
+            phdr_section->getShdr().sh_addr = phdr.p_vaddr + ehdr->e_ehsize;
+            segment->addSection(ehdr);
+            segment->addSection(phdr_section);
+            ehdr->markDirty();
+        }
+        if (phdr.p_type == PT_PHDR)
+            segment->addSection(phdr_section);
+        for (int j = 1; j < ehdr->e_shnum; j++)
+            if (phdr.contains(sections[j]))
+                segment->addSection(sections[j]);
+        // Make sure that our view of segments corresponds to the original
+        // ELF file.
+        assert(segment->getFileSize() == phdr.p_filesz);
+        assert(segment->getMemSize() == phdr.p_memsz);
+        segments.push_back(segment);
+    }
+
+    new (&eh_entry) ElfLocation(ehdr->e_entry, this);
+}
+
+Elf::~Elf()
+{
+    for (std::vector<ElfSegment *>::iterator seg = segments.begin(); seg != segments.end(); seg++)
+        delete *seg;
+    delete[] sections;
+    ElfSection *section = ehdr;
+    while (section != NULL) {
+        ElfSection *next = section->getNext();
+        delete section;
+        section = next;
+    }
+}
+
+// TODO: This shouldn't fail after inserting sections
+ElfSection *Elf::getSection(int index)
+{
+    if ((index < -1) || (index >= ehdr->e_shnum))
+        throw std::runtime_error("Section index out of bounds");
+    if (index == -1)
+        index = ehdr->e_shstrndx; // TODO: should be fixed to use the actual current number
+    // Special case: the section at index 0 is void
+    if (index == 0)
+        return NULL;
+    // Infinite recursion guard
+    if (sections[index] == (ElfSection *)this)
+        return NULL;
+    if (sections[index] == NULL) {
+        sections[index] = (ElfSection *)this;
+        switch (tmp_shdr[index]->sh_type) {
+        case SHT_DYNAMIC:
+            sections[index] = new ElfDynamic_Section(*tmp_shdr[index], tmp_file, this);
+            break;
+        case SHT_REL:
+            sections[index] = new ElfRel_Section<Elf_Rel>(*tmp_shdr[index], tmp_file, this);
+            break;
+        case SHT_RELA:
+            sections[index] = new ElfRel_Section<Elf_Rela>(*tmp_shdr[index], tmp_file, this);
+            break;
+        case SHT_SYMTAB:
+            sections[index] = new ElfSymtab_Section(*tmp_shdr[index], tmp_file, this);
+            break;
+        case SHT_STRTAB:
+            sections[index] = new ElfStrtab_Section(*tmp_shdr[index], tmp_file, this);
+            break;
+        default:
+            sections[index] = new ElfSection(*tmp_shdr[index], tmp_file, this);
+        }
+    }
+    return sections[index];
+}
+
+ElfSection *Elf::getSectionAt(unsigned int offset)
+{
+    for (int i = 1; i < ehdr->e_shnum; i++) {
+        ElfSection *section = getSection(i);
+        if ((section != NULL) && (section->getFlags() & SHF_ALLOC) && !(section->getFlags() & SHF_TLS) &&
+            (offset >= section->getAddr()) && (offset < section->getAddr() + section->getSize()))
+            return section;
+    }
+    return NULL;
+}
+
+ElfDynamic_Section *Elf::getDynSection()
+{
+    for (std::vector<ElfSegment *>::iterator seg = segments.begin(); seg != segments.end(); seg++)
+        if (((*seg)->getType() == PT_DYNAMIC) && ((*seg)->getFirstSection() != NULL) &&
+            (*seg)->getFirstSection()->getType() == SHT_DYNAMIC)
+            return (ElfDynamic_Section *)(*seg)->getFirstSection();
+
+    return NULL;
+}
+
+void Elf::write(std::ofstream &file)
+{
+    // fixup section headers sh_name; TODO: that should be done by sections
+    // themselves
+    for (ElfSection *section = ehdr; section != NULL; section = section->getNext()) {
+        if (section->getIndex() == 0)
+            continue;
+        else
+            ehdr->e_shnum = section->getIndex() + 1;
+        section->getShdr().sh_name = eh_shstrndx->getStrIndex(section->getName());
+    }
+    phdr_section->getNext()->markDirty();
+    // Adjust PT_LOAD segments
+    int i = 0;
+    for (std::vector<ElfSegment *>::iterator seg = segments.begin(); seg != segments.end(); seg++, i++) {
+        if ((*seg)->getType() == PT_LOAD) {
+            std::list<ElfSection *>::iterator it = (*seg)->begin();
+            for (ElfSection *last = *(it++); it != (*seg)->end(); last = *(it++)) {
+               if (((*it)->getType() != SHT_NOBITS) &&
+                   ((*it)->getAddr() - last->getAddr()) != ((*it)->getOffset() - last->getOffset())) {
+                   std::vector<ElfSegment *>::iterator next = seg;
+                   segments.insert(++next, (*seg)->splitBefore(*it));
+                   seg = segments.begin() + i;
+                   break;
+               }
+           }
+        }
+    }
+    // fixup ehdr before writing
+    if (ehdr->e_phnum != segments.size()) {
+        ehdr->e_phnum = segments.size();
+        phdr_section->getShdr().sh_size = segments.size() * Elf_Phdr::size(ehdr->e_ident[EI_CLASS]);
+        phdr_section->getNext()->markDirty();
+    }
+    // fixup shdr before writing
+    if (ehdr->e_shnum != shdr_section->getSize() / shdr_section->getEntSize())
+        shdr_section->getShdr().sh_size = ehdr->e_shnum * Elf_Shdr::size(ehdr->e_ident[EI_CLASS]);
+    ehdr->e_shoff = shdr_section->getOffset();
+    ehdr->e_entry = eh_entry.getValue();
+    ehdr->e_shstrndx = eh_shstrndx->getIndex();
+    for (ElfSection *section = ehdr;
+         section != NULL; section = section->getNext()) {
+        file.seekp(section->getOffset());
+        if (section == phdr_section) {
+            for (std::vector<ElfSegment *>::iterator seg = segments.begin(); seg != segments.end(); seg++) {
+                Elf_Phdr phdr;
+                phdr.p_type = (*seg)->getType();
+                phdr.p_flags = (*seg)->getFlags();
+                if ((*seg)->getFirstSection()) {
+                    phdr.p_offset = (*seg)->getFirstSection()->getOffset();
+                    phdr.p_vaddr = (*seg)->getFirstSection()->getAddr();
+                } else {
+                    phdr.p_offset = 0;
+                    phdr.p_vaddr = 0;
+                }
+                phdr.p_paddr = phdr.p_vaddr + (*seg)->getVPDiff();
+                phdr.p_filesz = (*seg)->getFileSize();
+                phdr.p_memsz = (*seg)->getMemSize();
+                phdr.p_align = (*seg)->getAlign();
+                phdr.serialize(file, ehdr->e_ident[EI_CLASS], ehdr->e_ident[EI_DATA]);
+            }
+        } else if (section == shdr_section) {
+            null_section.serialize(file, ehdr->e_ident[EI_CLASS], ehdr->e_ident[EI_DATA]);
+            for (ElfSection *sec = ehdr; sec!= NULL; sec = sec->getNext()) {
+                if (sec->getType() != SHT_NULL)
+                    sec->getShdr().serialize(file, ehdr->e_ident[EI_CLASS], ehdr->e_ident[EI_DATA]);
+            }
+        } else
+           section->serialize(file, ehdr->e_ident[EI_CLASS], ehdr->e_ident[EI_DATA]);
+    }
+}
+
+ElfSection::ElfSection(Elf_Shdr &s, std::ifstream *file, Elf *parent)
+: shdr(s),
+  link(shdr.sh_link == SHN_UNDEF ? NULL : parent->getSection(shdr.sh_link)),
+  next(NULL), previous(NULL), index(-1)
+{
+    if ((file == NULL) || (shdr.sh_type == SHT_NULL) || (shdr.sh_type == SHT_NOBITS))
+        data = NULL;
+    else {
+        data = new char[shdr.sh_size];
+        int pos = file->tellg();
+        file->seekg(shdr.sh_offset);
+        file->read(data, shdr.sh_size);
+        file->seekg(pos);
+    }
+    if (shdr.sh_name == 0)
+        name = NULL;
+    else {
+        ElfStrtab_Section *strtab = (ElfStrtab_Section *) parent->getSection(-1);
+        // Special case (see elfgeneric.cpp): if strtab is NULL, the
+        // section being created is the strtab.
+        if (strtab == NULL)
+            name = &data[shdr.sh_name];
+        else
+            name = strtab->getStr(shdr.sh_name);
+    }
+    // Only SHT_REL/SHT_RELA sections use sh_info to store a section
+    // number.
+    if ((shdr.sh_type == SHT_REL) || (shdr.sh_type == SHT_RELA))
+        info.section = shdr.sh_info ? parent->getSection(shdr.sh_info) : NULL;
+    else
+        info.index = shdr.sh_info;
+}
+
+unsigned int ElfSection::getAddr()
+{
+    if (shdr.sh_addr != (Elf32_Word)-1)
+        return shdr.sh_addr;
+
+    // It should be safe to adjust sh_addr for all allocated sections that
+    // are neither SHT_NOBITS nor SHT_PROGBITS
+    if ((previous != NULL) && isRelocatable()) {
+        unsigned int addr = previous->getAddr();
+        if (previous->getType() != SHT_NOBITS)
+            addr += previous->getSize();
+
+        if (addr & (getAddrAlign() - 1))
+            addr = (addr | (getAddrAlign() - 1)) + 1;
+
+        return (shdr.sh_addr = addr);
+    }
+    return shdr.sh_addr;
+}
+
+unsigned int ElfSection::getOffset()
+{
+    if (shdr.sh_offset != (Elf32_Word)-1)
+        return shdr.sh_offset;
+
+    if (previous == NULL)
+        return (shdr.sh_offset = 0);
+
+    unsigned int offset = previous->getOffset();
+    if (previous->getType() != SHT_NOBITS)
+        offset += previous->getSize();
+
+    // SHF_TLS is used for .tbss which is some kind of special case.
+    if (((getType() != SHT_NOBITS) || (getFlags() & SHF_TLS)) && (getFlags() & SHF_ALLOC)) {
+        if ((getAddr() & 4095) < (offset & 4095))
+            offset = (offset | 4095) + (getAddr() & 4095) + 1;
+        else
+            offset = (offset & ~4095) + (getAddr() & 4095);
+    }
+    // TODO: carefully handle this, as this isn't always safe, cf. when
+    // resizing .dynamic.
+    if ((getType() != SHT_NOBITS) && (offset & (getAddrAlign() - 1)))
+        offset = (offset | (getAddrAlign() - 1)) + 1;
+
+    return (shdr.sh_offset = offset);
+}
+
+int ElfSection::getIndex()
+{
+    if (index != -1)
+        return index;
+    if (getType() == SHT_NULL)
+        return (index = 0);
+    ElfSection *reference;
+    for (reference = previous; (reference != NULL) && (reference->getType() == SHT_NULL); reference = reference->getPrevious());
+    if (reference == NULL)
+        return (index = 1);
+    return (index = reference->getIndex() + 1);
+}
+
+Elf_Shdr &ElfSection::getShdr()
+{
+    getOffset();
+    if (shdr.sh_link == (Elf32_Word)-1)
+        shdr.sh_link = getLink() ? getLink()->getIndex() : 0;
+    if (shdr.sh_info == (Elf32_Word)-1)
+        shdr.sh_info = ((getType() == SHT_REL) || (getType() == SHT_RELA)) ?
+                       (getInfo().section ? getInfo().section->getIndex() : 0) :
+                       getInfo().index;
+
+    return shdr;
+}
+
+ElfSegment::ElfSegment(Elf_Phdr *phdr)
+: type(phdr->p_type), v_p_diff(phdr->p_paddr - phdr->p_vaddr),
+  flags(phdr->p_flags), align(phdr->p_align) {}
+
+void ElfSegment::addSection(ElfSection *section)
+{
+    //TODO: Check overlapping sections
+    std::list<ElfSection *>::iterator i;
+    for (i = sections.begin(); i != sections.end(); ++i)
+        if ((*i)->getAddr() > section->getAddr())
+            break;
+    sections.insert(i, section);
+}
+
+unsigned int ElfSegment::getFileSize()
+{
+    if (sections.empty())
+        return 0;
+    // Search the last section that is not SHT_NOBITS
+    std::list<ElfSection *>::reverse_iterator i;
+    for (i = sections.rbegin(); (i != sections.rend()) && ((*i)->getType() == SHT_NOBITS); ++i);
+    // All sections are SHT_NOBITS
+    if (i == sections.rend())
+        return 0;
+    return ((*i)->getAddr() - sections.front()->getAddr()) + (*i)->getSize();
+}
+
+unsigned int ElfSegment::getMemSize()
+{
+    if (sections.empty())
+        return 0;
+    return (sections.back()->getAddr() - sections.front()->getAddr()) +
+           (sections.back()->getSize());
+}
+
+ElfSegment *ElfSegment::splitBefore(ElfSection *section)
+{
+    std::list<ElfSection *>::iterator i, rm;
+    for (i = sections.begin(); (*i != section) && (i != sections.end()); ++i);
+    if (i == sections.end())
+        return NULL;
+
+    // Probably very wrong.
+    Elf_Phdr phdr;
+    phdr.p_type = type;
+    phdr.p_vaddr = 0;
+    phdr.p_paddr = phdr.p_vaddr + v_p_diff;
+    phdr.p_flags = flags;
+    phdr.p_align = 0x1000;
+    ElfSegment *segment = new ElfSegment(&phdr);
+
+    for (rm = i; i != sections.end(); ++i)
+        segment->addSection(*i);
+    sections.erase(rm, sections.end());
+
+    return segment;
+}
+
+ElfSection *ElfDynamic_Section::getSectionForType(unsigned int tag)
+{
+    for (unsigned int i = 0; i < shdr.sh_size / shdr.sh_entsize; i++)
+        if (dyns[i].tag == tag)
+            return dyns[i].value->getSection();
+
+    return NULL;
+}
+
+void ElfDynamic_Section::setValueForType(unsigned int tag, ElfValue *val)
+{
+    unsigned int i;
+    for (i = 0; (i < shdr.sh_size / shdr.sh_entsize) && (dyns[i].tag != DT_NULL); i++)
+        if (dyns[i].tag == tag) {
+            delete dyns[i].value;
+            dyns[i].value = val;
+            return;
+        }
+    // This should never happen, as the last entry is always tagged DT_NULL
+    assert(i < shdr.sh_size / shdr.sh_entsize);
+    // If we get here, this means we didn't match for the given tag
+    dyns[i].tag = tag;
+    dyns[i++].value = val;
+
+    // If we were on the last entry, we need to grow the section.
+    // Most of the time, though, there are a few DT_NULL entries.
+    if (i < shdr.sh_size / shdr.sh_entsize)
+        return;
+
+    Elf_DynValue value;
+    value.tag = DT_NULL;
+    value.value = NULL;
+    dyns.push_back(value);
+    // Resize the section accordingly
+    shdr.sh_size += shdr.sh_entsize;
+    if (getNext() != NULL)
+        getNext()->markDirty();
+}
+
+ElfDynamic_Section::ElfDynamic_Section(Elf_Shdr &s, std::ifstream *file, Elf *parent)
+: ElfSection(s, file, parent)
+{
+    int pos = file->tellg();
+    dyns.resize(s.sh_size / s.sh_entsize);
+    file->seekg(shdr.sh_offset);
+    // Here we assume tags refer to only one section (e.g. DT_RELSZ accounts
+    // for .rel.dyn size)
+    for (unsigned int i = 0; i < s.sh_size / s.sh_entsize; i++) {
+        Elf_Dyn dyn(*file, parent->getClass(), parent->getData());
+        dyns[i].tag = dyn.d_tag;
+        switch (dyn.d_tag) {
+        case DT_NULL:
+        case DT_SYMBOLIC:
+        case DT_TEXTREL:
+        case DT_BIND_NOW:
+            dyns[i].value = new ElfValue();
+            break;
+        case DT_NEEDED:
+        case DT_SONAME:
+        case DT_RPATH:
+        case DT_PLTREL:
+        case DT_RUNPATH:
+        case DT_FLAGS:
+        case DT_RELACOUNT:
+        case DT_RELCOUNT:
+        case DT_VERDEFNUM:
+        case DT_VERNEEDNUM:
+            dyns[i].value = new ElfPlainValue(dyn.d_un.d_val);
+            break;
+        case DT_PLTGOT:
+        case DT_HASH:
+        case DT_STRTAB:
+        case DT_SYMTAB:
+        case DT_RELA:
+        case DT_INIT:
+        case DT_FINI:
+        case DT_REL:
+        case DT_JMPREL:
+        case DT_INIT_ARRAY:
+        case DT_FINI_ARRAY:
+        case DT_GNU_HASH:
+        case DT_VERSYM:
+        case DT_VERNEED:
+        case DT_VERDEF:
+            dyns[i].value = new ElfLocation(dyn.d_un.d_ptr, parent);
+            break;
+        default:
+            dyns[i].value = NULL;
+        }
+    }
+    // Another loop to get the section sizes
+    for (unsigned int i = 0; i < s.sh_size / s.sh_entsize; i++)
+        switch (dyns[i].tag) {
+        case DT_PLTRELSZ:
+            dyns[i].value = new ElfSize(getSectionForType(DT_JMPREL));
+            break;
+        case DT_RELASZ:
+            dyns[i].value = new ElfSize(getSectionForType(DT_RELA));
+            break;
+        case DT_STRSZ:
+            dyns[i].value = new ElfSize(getSectionForType(DT_STRTAB));
+            break;
+        case DT_RELSZ:
+            dyns[i].value = new ElfSize(getSectionForType(DT_REL));
+            break;
+        case DT_INIT_ARRAYSZ:
+            dyns[i].value = new ElfSize(getSectionForType(DT_INIT_ARRAY));
+            break;
+        case DT_FINI_ARRAYSZ:
+            dyns[i].value = new ElfSize(getSectionForType(DT_FINI_ARRAY));
+            break;
+        case DT_RELAENT:
+            dyns[i].value = new ElfEntSize(getSectionForType(DT_RELA));
+            break;
+        case DT_SYMENT:
+            dyns[i].value = new ElfEntSize(getSectionForType(DT_SYMTAB));
+            break;
+        case DT_RELENT:
+            dyns[i].value = new ElfEntSize(getSectionForType(DT_REL));
+            break;
+        }
+
+    file->seekg(pos);
+}
+
+ElfDynamic_Section::~ElfDynamic_Section()
+{
+    for (unsigned int i = 0; i < shdr.sh_size / shdr.sh_entsize; i++)
+        delete dyns[i].value;
+}
+
+void ElfDynamic_Section::serialize(std::ofstream &file, char ei_class, char ei_data)
+{
+    for (unsigned int i = 0; i < shdr.sh_size / shdr.sh_entsize; i++) {
+        Elf_Dyn dyn;
+        dyn.d_tag = dyns[i].tag;
+        dyn.d_un.d_val = (dyns[i].value != NULL) ? dyns[i].value->getValue() : 0;
+        dyn.serialize(file, ei_class, ei_data);
+    }
+}
+
+ElfSymtab_Section::ElfSymtab_Section(Elf_Shdr &s, std::ifstream *file, Elf *parent)
+: ElfSection(s, file, parent)
+{
+    int pos = file->tellg();
+    file->seekg(shdr.sh_offset);
+    for (unsigned int i = 0; i < shdr.sh_size / shdr.sh_entsize; i++) {
+        Elf_Sym sym(*file, parent->getClass(), parent->getData());
+        syms.push_back(sym);
+    }
+    file->seekg(pos);
+}
+
+const char *
+ElfStrtab_Section::getStr(unsigned int index)
+{
+    for (std::vector<table_storage>::iterator t = table.begin();
+         t != table.end(); t++) {
+        if (index < t->used)
+            return t->buf + index;
+        index -= t->used;
+    }
+    assert(1 == 0);
+    return NULL;
+}
+
+const char *
+ElfStrtab_Section::getStr(const char *string)
+{
+    if (string == NULL)
+        return NULL;
+
+    // If the given string is within the section, return it
+    for (std::vector<table_storage>::iterator t = table.begin();
+         t != table.end(); t++)
+        if ((string >= t->buf) && (string < t->buf + t->used))
+            return string;
+
+    // TODO: should scan in the section to find an existing string
+
+    // If not, we need to allocate the string in the section
+    size_t len = strlen(string) + 1;
+
+    if (table.back().size - table.back().used < len)
+        table.resize(table.size() + 1);
+
+    char *alloc_str = table.back().buf + table.back().used;
+    memcpy(alloc_str, string, len);
+    table.back().used += len;
+
+    shdr.sh_size += len;
+    markDirty();
+
+    return alloc_str;
+}
+
+unsigned int
+ElfStrtab_Section::getStrIndex(const char *string)
+{
+    if (string == NULL)
+        return 0;
+
+    unsigned int index = 0;
+    string = getStr(string);
+    for (std::vector<table_storage>::iterator t = table.begin();
+         t != table.end(); t++) {
+        if ((string >= t->buf) && (string < t->buf + t->used))
+            return index + (string - t->buf);
+        index += t->used;
+    }
+
+    assert(1 == 0);
+    return 0;
+}
+
+void
+ElfStrtab_Section::serialize(std::ofstream &file, char ei_class, char ei_data)
+{
+    file.seekp(getOffset());
+    for (std::vector<table_storage>::iterator t = table.begin();
+         t != table.end(); t++)
+        file.write(t->buf, t->used);
+}

build/unix/elfhack/elfhack.cpp

@@ -0,0 +1,414 @@
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is elfhack.
+ *
+ * The Initial Developer of the Original Code is
+ * Mozilla Foundation.
+ * Portions created by the Initial Developer are Copyright (C) 2010
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   Mike Hommey <mh@glandium.org>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+#include <assert.h>
+#include <cstring>
+#include <cstdlib>
+#include <cstdio>
+#include "elfxx.h"
+
+#define ver "0"
+#define elfhack_data ".elfhack.data.v" ver
+#define elfhack_text ".elfhack.text.v" ver
+
+#ifndef R_ARM_V4BX
+#define R_ARM_V4BX 0x28
+#endif
+
+char *rundir = NULL;
+
+class Elf_RelHack_Traits {
+public:
+    typedef Elf32_Rel Type32;
+    typedef Elf32_Rel Type64;
+
+    template <class endian, typename R, typename T>
+    static inline void swap(T &t, R &r) {
+        r.r_offset = endian::swap(t.r_offset);
+        r.r_info = endian::swap(t.r_info);
+    }
+};
+
+typedef serializable<Elf_RelHack_Traits> Elf_RelHack;
+
+class ElfRelHack_Section: public ElfSection {
+public:
+    ElfRelHack_Section(Elf_Shdr &s)
+    : ElfSection(s, NULL, NULL)
+    {
+        name = elfhack_data;
+    };
+
+    void serialize(std::ofstream &file, char ei_class, char ei_data)
+    {
+        for (std::vector<Elf_RelHack>::iterator i = rels.begin();
+             i != rels.end(); ++i)
+            (*i).serialize(file, ei_class, ei_data);
+    }
+
+    bool isRelocatable() {
+        return true;
+    }
+
+    void push_back(Elf_RelHack &r) {
+        rels.push_back(r);
+        shdr.sh_size = rels.size() * shdr.sh_entsize;
+    }
+private:
+    std::vector<Elf_RelHack> rels;
+};
+
+class ElfRelHackCode_Section: public ElfSection {
+public:
+    ElfRelHackCode_Section(Elf_Shdr &s, Elf &e)
+    : ElfSection(s, NULL, NULL), parent(e) {
+        std::string file(rundir);
+        init = parent.getDynSection()->getSectionForType(DT_INIT);
+        file += "/inject/";
+        switch (parent.getMachine()) {
+        case EM_386:
+            file += "x86";
+            break;
+        case EM_X86_64:
+            file += "x86_64";
+            break;
+        case EM_ARM:
+            file += "arm";
+            break;
+        default:
+            throw std::runtime_error("unsupported architecture");
+        }
+        if (init == NULL)
+            file += "-noinit";
+        file += ".o";
+        std::ifstream inject(file.c_str(), std::ios::in|std::ios::binary);
+        elf = new Elf(inject);
+        if (elf->getType() != ET_REL)
+            throw std::runtime_error("object for injected code is not ET_REL");
+        if (elf->getMachine() != parent.getMachine())
+            throw std::runtime_error("architecture of object for injected code doesn't match");
+
+        // Get all executable sections from the injected code object.
+        // Most of the time, there will only be one for the init function,
+        // but on e.g. x86, there is a separate section for
+        // __i686.get_pc_thunk.$reg
+        for (ElfSection *text = elf->getSection(1); text != NULL;
+             text = text->getNext()) {
+            if ((text->getType() == SHT_PROGBITS) &&
+                (text->getFlags() & SHF_EXECINSTR)) {
+                code.push_back(text);
+                // We need to align this section depending on the greater
+                // alignment required by code sections.
+                if (shdr.sh_addralign < text->getAddrAlign())
+                    shdr.sh_addralign = text->getAddrAlign();
+            }
+        }
+        assert(code.size() != 0);
+
+        // Adjust code sections offsets according to their size
+        std::vector<ElfSection *>::iterator c = code.begin();
+        (*c)->getShdr().sh_addr = 0;
+        for(ElfSection *last = *(c++); c != code.end(); c++) {
+            unsigned int addr = last->getShdr().sh_addr + last->getSize();
+            if (addr & ((*c)->getAddrAlign() - 1))
+                addr = (addr | ((*c)->getAddrAlign() - 1)) + 1;
+            (*c)->getShdr().sh_addr = addr;
+        }
+        shdr.sh_size = code.back()->getAddr() + code.back()->getSize();
+        data = new char[shdr.sh_size];
+        char *buf = data;
+        for (c = code.begin(); c != code.end(); c++) {
+            memcpy(buf, (*c)->getData(), (*c)->getSize());
+            buf += (*c)->getSize();
+        }
+        name = elfhack_text;
+    }
+
+    ~ElfRelHackCode_Section() {
+        delete elf;
+    }
+
+    void serialize(std::ofstream &file, char ei_class, char ei_data)
+    {
+        // Readjust code offsets
+        for (std::vector<ElfSection *>::iterator c = code.begin(); c != code.end(); c++)
+            (*c)->getShdr().sh_addr += getAddr();
+
+        // Apply relocations
+        for (ElfSection *rel = elf->getSection(1); rel != NULL; rel = rel->getNext())
+            if ((rel->getType() == SHT_REL) || (rel->getType() == SHT_RELA)) {
+                ElfSection *section = rel->getInfo().section;
+                if ((section->getType() == SHT_PROGBITS) && (section->getFlags() & SHF_EXECINSTR)) {
+                    if (rel->getType() == SHT_REL)
+                        apply_relocations((ElfRel_Section<Elf_Rel> *)rel, section);
+                    else
+                        apply_relocations((ElfRel_Section<Elf_Rela> *)rel, section);
+                }
+            }
+
+        ElfSection::serialize(file, ei_class, ei_data);
+    }
+
+    bool isRelocatable() {
+        return true;
+    }
+
+private:
+    void apply_pc32_relocation(ElfSection *the_code, char *base, Elf_Rel *r, unsigned int addr)
+    {
+        *(Elf32_Addr *)(base + r->r_offset) += (Elf32_Addr) (addr - r->r_offset - the_code->getAddr());
+    }
+
+    void apply_pc32_relocation(ElfSection *the_code, char *base, Elf_Rela *r, unsigned int addr)
+    {
+        *(Elf32_Addr *)(base + r->r_offset) = (Elf32_Addr) (addr + r->r_addend - r->r_offset - the_code->getAddr());
+    }
+
+    void apply_arm_plt32_relocation(ElfSection *the_code, char *base, Elf_Rel *r, unsigned int addr)
+    {
+        // We don't care about sign_extend because the only case where this is
+        // going to be used only jumps forward.
+        Elf32_Addr addend = (Elf32_Addr) (addr - r->r_offset - the_code->getAddr()) >> 2;
+        addend = (*(Elf32_Addr *)(base + r->r_offset) + addend) & 0x00ffffff;
+        *(Elf32_Addr *)(base + r->r_offset) = (*(Elf32_Addr *)(base + r->r_offset) & 0xff000000) | addend;
+    }
+
+    void apply_arm_plt32_relocation(ElfSection *the_code, char *base, Elf_Rela *r, unsigned int addr)
+    {
+        // We don't care about sign_extend because the only case where this is
+        // going to be used only jumps forward.
+        Elf32_Addr addend = (Elf32_Addr) (addr - r->r_offset - the_code->getAddr()) >> 2;
+        addend = (r->r_addend + addend) & 0x00ffffff;
+        *(Elf32_Addr *)(base + r->r_offset) = (r->r_addend & 0xff000000) | addend;
+    }
+
+    void apply_gotoff_relocation(ElfSection *the_code, char *base, Elf_Rel *r, unsigned int addr)
+    {
+        *(Elf32_Addr *)(base + r->r_offset) += (Elf32_Addr) addr;
+    }
+
+    void apply_gotoff_relocation(ElfSection *the_code, char *base, Elf_Rela *r, unsigned int addr)
+    {
+        *(Elf32_Addr *)(base + r->r_offset) = (Elf32_Addr) addr + r->r_addend;
+    }
+
+    template <typename Rel_Type>
+    void apply_relocations(ElfRel_Section<Rel_Type> *rel, ElfSection *the_code)
+    {
+        assert(rel->getType() == Rel_Type::sh_type);
+        char *buf = data + (the_code->getAddr() - code.front()->getAddr());
+        // TODO: various checks on the sections
+        ElfSymtab_Section *symtab = (ElfSymtab_Section *)rel->getLink();
+        ElfStrtab_Section *strtab = (ElfStrtab_Section *)symtab->getLink();
+        for (typename std::vector<Rel_Type>::iterator r = rel->rels.begin(); r != rel->rels.end(); r++) {
+            // TODO: various checks on the symbol
+            const char *name = strtab->getStr(symtab->syms[ELF32_R_SYM(r->r_info)].st_name);
+            unsigned int addr;
+            if (symtab->syms[ELF32_R_SYM(r->r_info)].st_shndx == 0) {
+                if (strcmp(name, "relhack") == 0) {
+                    addr = getNext()->getAddr();
+                } else if (strcmp(name, "elf_header") == 0) {
+                    // TODO: change this ungly hack to something better
+                    ElfSection *ehdr = parent.getSection(1)->getPrevious()->getPrevious();
+                    addr = ehdr->getAddr();
+                } else if (strcmp(name, "original_init") == 0) {
+                    addr = init->getAddr();
+                } else if (strcmp(name, "_GLOBAL_OFFSET_TABLE_") == 0) {
+                    // We actually don't need a GOT, but need it as a reference for
+                    // GOTOFF relocations. We'll just use the start of the ELF file
+                    addr = 0;
+                } else if (strcmp(name, "") == 0) {
+                    // This is for R_ARM_V4BX, until we find something better
+                    addr = -1;
+                } else {
+                    fprintf(stderr, "Unsupported symbol in relocation: %s\n", name);
+                    break;
+                }
+            } else {
+                ElfSection *section = elf->getSection(symtab->syms[ELF32_R_SYM(r->r_info)].st_shndx);
+                assert((section->getType() == SHT_PROGBITS) && (section->getFlags() & SHF_EXECINSTR));
+                addr = section->getAddr() + symtab->syms[ELF32_R_SYM(r->r_info)].st_value;
+            }
+            // Do the relocation
+#define REL(machine, type) (EM_ ## machine | (R_ ## machine ## _ ## type << 8))
+            switch (elf->getMachine() | (ELF32_R_TYPE(r->r_info) << 8)) {
+            case REL(X86_64, PC32):
+            case REL(386, PC32):
+            case REL(386, GOTPC):
+            case REL(ARM, GOTPC):
+                apply_pc32_relocation(the_code, buf, &*r, addr);
+                break;
+            case REL(ARM, PLT32):
+                apply_arm_plt32_relocation(the_code, buf, &*r, addr);
+                break;
+            case REL(386, GOTOFF):
+            case REL(ARM, GOTOFF):
+                apply_gotoff_relocation(the_code, buf, &*r, addr);
+                break;
+            case REL(ARM, V4BX):
+                // Ignore R_ARM_V4BX relocations
+                break;
+            default:
+                fprintf(stderr, "Unsupported relocation type\n");
+            }
+        }
+    }
+
+    Elf *elf, &parent;
+    std::vector<ElfSection *> code;
+    ElfSection *init;
+};
+
+template <typename Rel_Type>
+int do_relocation_section(Elf *elf, unsigned int rel_type)
+{
+    ElfDynamic_Section *dyn = elf->getDynSection();
+    if (dyn ==NULL) {
+        fprintf(stderr, "Couldn't find SHT_DYNAMIC section\n");
+        return -1;
+    }
+
+    ElfRel_Section<Rel_Type> *section = (ElfRel_Section<Rel_Type> *)dyn->getSectionForType(Rel_Type::d_tag);
+    assert(section->getType() == Rel_Type::sh_type);
+
+    Elf32_Shdr relhack32_section =
+        { 0, SHT_PROGBITS, SHF_ALLOC, 0, -1, 0, SHN_UNDEF, 0,
+          Elf_RelHack::size(elf->getClass()), Elf_RelHack::size(elf->getClass()) }; // TODO: sh_addralign should be an alignment, not size
+    Elf32_Shdr relhackcode32_section =
+        { 0, SHT_PROGBITS, SHF_ALLOC | SHF_EXECINSTR, 0, -1, 0, SHN_UNDEF, 0, 1, 0 };
+    Elf_Shdr relhack_section(relhack32_section);
+    Elf_Shdr relhackcode_section(relhackcode32_section);
+    ElfRelHack_Section *relhack = new ElfRelHack_Section(relhack_section);
+    ElfRelHackCode_Section *relhackcode = new ElfRelHackCode_Section(relhackcode_section, *elf);
+    relhackcode->insertAfter(section);
+    relhack->insertAfter(relhackcode);
+
+    std::vector<Rel_Type> new_rels;
+    Elf_RelHack relhack_entry;
+    relhack_entry.r_offset = relhack_entry.r_info = 0;
+    int entry_sz = (elf->getClass() == ELFCLASS32) ? 4 : 8;
+    for (typename std::vector<Rel_Type>::iterator i = section->rels.begin();
+         i != section->rels.end(); i++) {
+        // Don't pack relocations happening in non writable sections.
+        // Our injected code is likely not to be allowed to write there.
+        ElfSection *section = elf->getSectionAt(i->r_offset);
+        if (!(section->getFlags() & SHF_WRITE) || (ELF32_R_TYPE(i->r_info) != rel_type))
+            new_rels.push_back(*i);
+        else {
+            // TODO: check that i->r_addend == *i->r_offset
+            if (i->r_offset == relhack_entry.r_offset + relhack_entry.r_info * entry_sz) {
+                relhack_entry.r_info++;
+            } else {
+                if (relhack_entry.r_offset)
+                    relhack->push_back(relhack_entry);
+                relhack_entry.r_offset = i->r_offset;
+                relhack_entry.r_info = 1;
+            }
+        }
+    }
+    if (relhack_entry.r_offset)
+        relhack->push_back(relhack_entry);
+    // Last entry must be NULL
+    relhack_entry.r_offset = relhack_entry.r_info = 0;
+    relhack->push_back(relhack_entry);
+
+    section->rels.assign(new_rels.begin(), new_rels.end());
+    section->shrink(new_rels.size() * section->getEntSize());
+    ElfLocation *init = new ElfLocation(relhackcode, 0);
+    dyn->setValueForType(DT_INIT, init);
+    // TODO: adjust the value according to the remaining number of relative relocations
+    dyn->setValueForType(Rel_Type::d_tag_count, new ElfPlainValue(0));
+    return 0;
+}
+
+static inline int backup_file(const char *name)
+{
+    std::string fname(name);
+    fname += ".bak";
+    return rename(name, fname.c_str());
+}
+
+void do_file(const char *name, bool backup = false)
+{
+    std::ifstream file(name, std::ios::in|std::ios::binary);
+    Elf *elf = new Elf(file);
+    unsigned int size = elf->getSize();
+    fprintf(stderr, "%s: ", name);
+
+    int exit = -1;
+    switch (elf->getMachine()) {
+    case EM_386:
+        exit = do_relocation_section<Elf_Rel>(elf, R_386_RELATIVE);
+        break;
+    case EM_X86_64:
+        exit = do_relocation_section<Elf_Rela>(elf, R_X86_64_RELATIVE);
+        break;
+    case EM_ARM:
+        exit = do_relocation_section<Elf_Rel>(elf, R_ARM_RELATIVE);
+        break;
+    }
+    if (elf->getSize() >= size)
+        fprintf(stderr, "No gain. Aborting\n");
+    else if (exit == 0) {
+        if (backup && backup_file(name) != 0) {
+            fprintf(stderr, "Couln't create backup file\n");
+        } else {
+            std::ofstream ofile(name, std::ios::out|std::ios::binary|std::ios::trunc);
+            elf->write(ofile);
+            fprintf(stderr, "Reduced by %d bytes\n", size - elf->getSize());
+        }
+    }
+    delete elf;
+}
+
+int main(int argc, char *argv[])
+{
+    int arg;
+    bool backup = false;
+    char *lastSlash = rindex(argv[0], '/');
+    if (lastSlash != NULL)
+        rundir = strndup(argv[0], lastSlash - argv[0]);
+    for (arg = 1; arg < argc; arg++) {
+        if (strcmp(argv[arg], "-b") == 0)
+            backup = true;
+        else
+            do_file(argv[arg], backup);
+    }
+
+    free(rundir);
+    return 0;
+}

build/unix/elfhack/elfxx.h

@@ -0,0 +1,582 @@
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is elfhack.
+ *
+ * The Initial Developer of the Original Code is
+ * Mozilla Foundation.
+ * Portions created by the Initial Developer are Copyright (C) 2010
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   Mike Hommey <mh@glandium.org>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+#include <stdexcept>
+#include <list>
+#include <vector>
+#include <cstring>
+#include <iostream>
+#include <fstream>
+#include <elf.h>
+#include <asm/byteorder.h>
+
+// Technically, __*_to_cpu and __cpu_to* function are equivalent,
+// so swap can use either of both.
+#define def_swap(endian, type, bits) \
+static inline type ## bits ## _t swap(type ## bits ## _t i) { \
+    return __ ## endian ## bits ## _to_cpu(i); \
+}
+
+class little_endian {
+public:
+def_swap(le, uint, 16);
+def_swap(le, uint, 32);
+def_swap(le, uint, 64);
+def_swap(le, int, 16);
+def_swap(le, int, 32);
+def_swap(le, int, 64);
+};
+
+class big_endian {
+public:
+def_swap(be, uint, 16);
+def_swap(be, uint, 32);
+def_swap(be, uint, 64);
+def_swap(be, int, 16);
+def_swap(be, int, 32);
+def_swap(be, int, 64);
+};
+
+// forward declaration
+class ElfSection;
+class ElfSegment;
+// TODO: Rename Elf_* types
+class Elf_Ehdr;
+class Elf_Phdr;
+class Elf;
+class ElfDynamic_Section;
+class ElfStrtab_Section;
+
+class Elf_Ehdr_Traits {
+public:
+    typedef Elf32_Ehdr Type32;
+    typedef Elf64_Ehdr Type64;
+
+    template <class endian, typename R, typename T>
+    static void swap(T &t, R &r);
+};
+
+class Elf_Phdr_Traits {
+public:
+    typedef Elf32_Phdr Type32;
+    typedef Elf64_Phdr Type64;
+
+    template <class endian, typename R, typename T>
+    static void swap(T &t, R &r);
+};
+
+class Elf_Shdr_Traits {
+public:
+    typedef Elf32_Shdr Type32;
+    typedef Elf64_Shdr Type64;
+
+    template <class endian, typename R, typename T>
+    static void swap(T &t, R &r);
+};
+
+class Elf_Dyn_Traits {
+public:
+    typedef Elf32_Dyn Type32;
+    typedef Elf64_Dyn Type64;
+
+    template <class endian, typename R, typename T>
+    static void swap(T &t, R &r);
+};
+
+class Elf_Sym_Traits {
+public:
+    typedef Elf32_Sym Type32;
+    typedef Elf64_Sym Type64;
+
+    template <class endian, typename R, typename T>
+    static void swap(T &t, R &r);
+};
+
+class Elf_Rel_Traits {
+public:
+    typedef Elf32_Rel Type32;
+    typedef Elf64_Rel Type64;
+
+    template <class endian, typename R, typename T>
+    static void swap(T &t, R &r);
+};
+
+class Elf_Rela_Traits {
+public:
+    typedef Elf32_Rela Type32;
+    typedef Elf64_Rela Type64;
+
+    template <class endian, typename R, typename T>
+    static void swap(T &t, R &r);
+};
+
+class ElfValue {
+public:
+    virtual unsigned int getValue() { return 0; }
+    virtual ElfSection *getSection() { return NULL; }
+};
+
+class ElfPlainValue: public ElfValue {
+    unsigned int value;
+public:
+    ElfPlainValue(unsigned int val): value(val) {};
+    unsigned int getValue() { return value; }
+};
+
+class ElfLocation: public ElfValue {
+    ElfSection *section;
+    unsigned int offset;
+public:
+    ElfLocation(): section(NULL), offset(0) {};
+    ElfLocation(ElfSection *section, unsigned int offset): section(section), offset(offset) {};
+    ElfLocation(unsigned int location, Elf *elf);
+    unsigned int getValue();
+    ElfSection *getSection() { return section; }
+};
+
+class ElfSize: public ElfValue {
+    ElfSection *section;
+public:
+    ElfSize(ElfSection *s): section(s) {};
+    unsigned int getValue();
+    ElfSection *getSection() { return section; }
+};
+
+class ElfEntSize: public ElfValue {
+    ElfSection *section;
+public:
+    ElfEntSize(ElfSection *s): section(s) {};
+    unsigned int getValue();
+    ElfSection *getSection() { return section; }
+};
+
+template <typename T>
+class serializable: public T::Type32 {
+public:
+    serializable() {};
+    serializable(const typename T::Type32 &p): T::Type32(p) {};
+    serializable(std::ifstream &file, char ei_class, char ei_data)
+    {
+        if (ei_class == ELFCLASS32) {
+            typename T::Type32 e;
+            file.read((char *)&e, sizeof(e));
+            if (ei_data == ELFDATA2LSB) {
+                T::template swap<little_endian>(e, *this);
+                return;
+            } else if (ei_data == ELFDATA2MSB) {
+                T::template swap<big_endian>(e, *this);
+                return;
+            }
+        } else if (ei_class == ELFCLASS64) {
+            typename T::Type64 e;
+            file.read((char *)&e, sizeof(e));
+            if (ei_data == ELFDATA2LSB) {
+                T::template swap<little_endian>(e, *this);
+                return;
+            } else if (ei_data == ELFDATA2MSB) {
+                T::template swap<big_endian>(e, *this);
+                return;
+            }
+        }
+        throw std::runtime_error("Unsupported ELF class or data encoding");
+    }
+
+    void serialize(std::ofstream &file, char ei_class, char ei_data)
+    {
+        if (ei_class == ELFCLASS32) {
+            typename T::Type32 e;
+            if (ei_data == ELFDATA2LSB) {
+                T::template swap<little_endian>(*this, e);
+                file.write((char *)&e, sizeof(e));
+                return;
+            } else if (ei_data == ELFDATA2MSB) {
+                T::template swap<big_endian>(*this, e);
+                file.write((char *)&e, sizeof(e));
+                return;
+            }
+        } else if (ei_class == ELFCLASS64) {
+            typename T::Type64 e;
+            if (ei_data == ELFDATA2LSB) {
+                T::template swap<little_endian>(*this, e);
+                file.write((char *)&e, sizeof(e));
+                return;
+            } else if (ei_data == ELFDATA2MSB) {
+                T::template swap<big_endian>(*this, e);
+                file.write((char *)&e, sizeof(e));
+                return;
+            }
+        }
+        throw std::runtime_error("Unsupported ELF class or data encoding");
+    }
+
+    static inline int size(char ei_class)
+    {
+        if (ei_class == ELFCLASS32)
+            return sizeof(typename T::Type32);
+        else if (ei_class == ELFCLASS64)
+            return sizeof(typename T::Type64);
+        return 0;
+    }
+};
+
+typedef serializable<Elf_Shdr_Traits> Elf_Shdr;
+
+class Elf {
+public:
+    Elf(std::ifstream &file);
+    ~Elf();
+
+    /* index == -1 is treated as index == ehdr.e_shstrndx */
+    ElfSection *getSection(int index);
+
+    ElfSection *getSectionAt(unsigned int offset);
+
+    ElfDynamic_Section *getDynSection();
+
+    void write(std::ofstream &file);
+
+    char getClass();
+    char getData();
+    char getType();
+    char getMachine();
+    unsigned int getSize();
+private:
+    Elf_Ehdr *ehdr;
+    ElfLocation eh_entry;
+    ElfStrtab_Section *eh_shstrndx;
+    ElfSection **sections;
+    std::vector<ElfSegment *> segments;
+    ElfSection *shdr_section, *phdr_section;
+    /* Values used only during initialization */
+    Elf_Shdr **tmp_shdr;
+    std::ifstream *tmp_file;
+};
+
+class ElfSection {
+public:
+    typedef union {
+        ElfSection *section;
+        int index;
+    } SectionInfo;
+
+    ElfSection(Elf_Shdr &s, std::ifstream *file, Elf *parent);
+
+    virtual ~ElfSection() {
+        delete[] data;
+    }
+
+    const char *getName() { return name; }
+    unsigned int getType() { return shdr.sh_type; }
+    unsigned int getFlags() { return shdr.sh_flags; }
+    unsigned int getAddr();
+    unsigned int getSize() { return shdr.sh_size; }
+    unsigned int getAddrAlign() { return shdr.sh_addralign; }
+    unsigned int getEntSize() { return shdr.sh_entsize; }
+    const char *getData() { return data; }
+    ElfSection *getLink() { return link; }
+    SectionInfo getInfo() { return info; }
+
+    void shrink(unsigned int newsize) {
+        if (newsize < shdr.sh_size) {
+            shdr.sh_size = newsize;
+            if (next)
+                next->markDirty();
+        }
+    }
+
+    unsigned int getOffset();
+    int getIndex();
+    Elf_Shdr &getShdr();
+
+    ElfSection *getNext() { return next; }
+    ElfSection *getPrevious() { return previous; }
+
+    virtual bool isRelocatable() {
+        return ((getType() != SHT_NULL) &&
+                (getType() != SHT_NOBITS) &&
+                (getType() != SHT_PROGBITS) &&
+                (getFlags() == SHF_ALLOC));
+    }
+
+    void insertAfter(ElfSection *section) {
+        if (previous != NULL)
+            previous->next = next;
+        if (next != NULL)
+            next->previous = previous;
+        previous = section;
+        if (section != NULL) {
+            next = section->next;
+            section->next = this;
+        } else
+            next = NULL;
+        if (next != NULL)
+            next->previous = this;
+        markDirty();
+    }
+
+    void markDirty() {
+        if (link != NULL)
+            shdr.sh_link = -1;
+        if (info.index)
+            shdr.sh_info = -1;
+        shdr.sh_offset = -1;
+        if (isRelocatable())
+            shdr.sh_addr = -1;
+        if (next)
+            next->markDirty();
+    }
+
+    virtual void serialize(std::ofstream &file, char ei_class, char ei_data)
+    {
+        if (getType() == SHT_NOBITS)
+            return;
+        file.seekp(getOffset());
+        file.write(data, getSize());
+    }
+protected:
+    Elf_Shdr shdr;
+    char *data;
+    const char *name;
+private:
+    ElfSection *link;
+    SectionInfo info;
+    ElfSection *next, *previous;
+    int index;
+};
+
+class ElfSegment {
+public:
+    ElfSegment(Elf_Phdr *phdr);
+
+    unsigned int getType() { return type; }
+    unsigned int getFlags() { return flags; }
+    unsigned int getAlign() { return type == PT_LOAD ? 0x1000 : align; /* TODO: remove this gross hack */ }
+
+    ElfSection *getFirstSection() { return sections.empty() ? NULL : sections.front(); }
+    int getVPDiff() { return v_p_diff; }
+    unsigned int getFileSize();
+    unsigned int getMemSize();
+
+    void addSection(ElfSection *section);
+
+    std::list<ElfSection *>::iterator begin() { return sections.begin(); }
+    std::list<ElfSection *>::iterator end() { return sections.end(); }
+
+    ElfSegment *splitBefore(ElfSection *section);
+private:
+    unsigned int type;
+    int v_p_diff; // Difference between physical and virtual address
+    unsigned int flags;
+    unsigned int align;
+    std::list<ElfSection *> sections;
+};
+
+class Elf_Ehdr: public serializable<Elf_Ehdr_Traits>, public ElfSection {
+public:
+    Elf_Ehdr(std::ifstream &file, char ei_class, char ei_data);
+    void serialize(std::ofstream &file, char ei_class, char ei_data)
+    {
+        serializable<Elf_Ehdr_Traits>::serialize(file, ei_class, ei_data);
+    }
+};
+
+class Elf_Phdr: public serializable<Elf_Phdr_Traits> {
+public:
+    Elf_Phdr() {};
+    Elf_Phdr(std::ifstream &file, char ei_class, char ei_data)
+    : serializable<Elf_Phdr_Traits>(file, ei_class, ei_data) {};
+    bool contains(ElfSection *section)
+    {
+        unsigned int size = section->getSize();
+        unsigned int addr = section->getAddr();
+        // This may be biased, but should work in most cases
+        if ((section->getFlags() & SHF_ALLOC) == 0)
+            return false;
+        return (addr >= p_vaddr) &&
+               (addr + size <= p_vaddr + p_memsz);
+
+    }
+};
+
+typedef serializable<Elf_Dyn_Traits> Elf_Dyn;
+
+struct Elf_DynValue {
+    unsigned int tag;
+    ElfValue *value;
+};
+
+class ElfDynamic_Section: public ElfSection {
+public:
+    ElfDynamic_Section(Elf_Shdr &s, std::ifstream *file, Elf *parent);
+    ~ElfDynamic_Section();
+
+    void serialize(std::ofstream &file, char ei_class, char ei_data);
+
+    ElfSection *getSectionForType(unsigned int tag);
+    void setValueForType(unsigned int tag, ElfValue *val);
+private:
+    std::vector<Elf_DynValue> dyns;
+};
+
+typedef serializable<Elf_Sym_Traits> Elf_Sym;
+
+class ElfSymtab_Section: public ElfSection {
+public:
+    ElfSymtab_Section(Elf_Shdr &s, std::ifstream *file, Elf *parent);
+
+//private: // Until we have a real API
+    std::vector<Elf_Sym> syms;
+};
+
+class Elf_Rel: public serializable<Elf_Rel_Traits> {
+public:
+    Elf_Rel(std::ifstream &file, char ei_class, char ei_data)
+    : serializable<Elf_Rel_Traits>(file, ei_class, ei_data) {};
+
+    static const unsigned int sh_type = SHT_REL;
+    static const unsigned int d_tag = DT_REL;
+    static const unsigned int d_tag_count = DT_RELCOUNT;
+};
+
+class Elf_Rela: public serializable<Elf_Rela_Traits> {
+public:
+    Elf_Rela(std::ifstream &file, char ei_class, char ei_data)
+    : serializable<Elf_Rela_Traits>(file, ei_class, ei_data) {};
+
+    static const unsigned int sh_type = SHT_RELA;
+    static const unsigned int d_tag = DT_RELA;
+    static const unsigned int d_tag_count = DT_RELACOUNT;
+};
+
+template <class Rel>
+class ElfRel_Section: public ElfSection {
+public:
+    ElfRel_Section(Elf_Shdr &s, std::ifstream *file, Elf *parent)
+    : ElfSection(s, file, parent)
+    {
+        int pos = file->tellg();
+        file->seekg(shdr.sh_offset);
+        for (unsigned int i = 0; i < s.sh_size / s.sh_entsize; i++) {
+            Rel r(*file, parent->getClass(), parent->getData());
+            rels.push_back(r);
+        }
+        file->seekg(pos);
+    }
+
+    void serialize(std::ofstream &file, char ei_class, char ei_data)
+    {
+        for (typename std::vector<Rel>::iterator i = rels.begin();
+             i != rels.end(); ++i)
+            (*i).serialize(file, ei_class, ei_data);
+    }
+//private: // Until we have a real API
+    std::vector<Rel> rels;
+};
+
+class ElfStrtab_Section: public ElfSection {
+public:
+    ElfStrtab_Section(Elf_Shdr &s, std::ifstream *file, Elf *parent)
+    : ElfSection(s, file, parent)
+    {
+        table.push_back(table_storage(data, shdr.sh_size));
+    }
+
+    ~ElfStrtab_Section()
+    {
+        for (std::vector<table_storage>::iterator t = table.begin() + 1;
+             t != table.end(); t++)
+            delete[] t->buf;
+    }
+
+    const char *getStr(unsigned int index);
+
+    const char *getStr(const char *string);
+
+    unsigned int getStrIndex(const char *string);
+
+    void serialize(std::ofstream &file, char ei_class, char ei_data);
+private:
+    struct table_storage {
+        unsigned int size, used;
+        char *buf;
+
+        table_storage(): size(4096), used(0), buf(new char[4096]) {}
+        table_storage(const char *data, unsigned int sz)
+        : size(sz), used(sz), buf(const_cast<char *>(data)) {}
+    };
+    std::vector<table_storage> table;
+};
+
+inline char Elf::getClass() {
+    return ehdr->e_ident[EI_CLASS];
+}
+
+inline char Elf::getData() {
+    return ehdr->e_ident[EI_DATA];
+}
+
+inline char Elf::getType() {
+    return ehdr->e_type;
+}
+
+inline char Elf::getMachine() {
+    return ehdr->e_machine;
+}
+
+inline unsigned int Elf::getSize() {
+    ElfSection *section;
+    for (section = shdr_section /* It's usually not far from the end */;
+        section->getNext() != NULL; section = section->getNext());
+    return section->getOffset() + section->getSize();
+}
+
+inline ElfLocation::ElfLocation(unsigned int location, Elf *elf) {
+    section = elf->getSectionAt(location);
+    offset = location - section->getAddr();
+}
+
+inline unsigned int ElfLocation::getValue() {
+    return section->getAddr() + offset;
+}
+
+inline unsigned int ElfSize::getValue() {
+    return section->getSize();
+}
+
+inline unsigned int ElfEntSize::getValue() {
+    return section->getEntSize();
+}

build/unix/elfhack/inject.c

@@ -0,0 +1,71 @@
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is elfhack.
+ *
+ * The Initial Developer of the Original Code is
+ * Mozilla Foundation.
+ * Portions created by the Initial Developer are Copyright (C) 2010
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   Mike Hommey <mh@glandium.org>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+#include <stdint.h>
+#include <elf.h>
+
+/* The Android NDK headers define those */
+#undef Elf_Ehdr
+#undef Elf_Addr
+
+#if BITS == 32
+#define Elf_Ehdr Elf32_Ehdr
+#define Elf_Addr Elf32_Addr
+#else
+#define Elf_Ehdr Elf64_Ehdr
+#define Elf_Addr Elf64_Addr
+#endif
+
+extern __attribute__((visibility("hidden"))) void original_init(int argc, char **argv, char **env);
+
+extern __attribute__((visibility("hidden"))) Elf32_Rel relhack[];
+extern __attribute__((visibility("hidden"))) Elf_Ehdr elf_header;
+
+void init(int argc, char **argv, char **env)
+{
+    Elf32_Rel *rel;
+    Elf_Addr *ptr, *start;
+    for (rel = relhack; rel->r_offset; rel++) {
+        start = (Elf_Addr *)((intptr_t)&elf_header + rel->r_offset);
+        for (ptr = start; ptr < &start[rel->r_info]; ptr++)
+            *ptr += (intptr_t)&elf_header;
+    }
+
+#ifndef NOINIT
+    original_init(argc, argv, env);
+#endif
+}

build/unix/elfhack/test.c

@@ -0,0 +1,148 @@
+#ifdef DEF
+DEF(This)
+DEF(is)
+DEF(a)
+DEF(test)
+DEF(of)
+DEF(string)
+DEF(array)
+DEF(for)
+DEF(use)
+DEF(with)
+DEF(elfhack)
+DEF(to)
+DEF(see)
+DEF(whether)
+DEF(it)
+DEF(breaks)
+DEF(anything)
+DEF(but)
+DEF(one)
+DEF(needs)
+DEF(quite)
+DEF(some)
+DEF(strings)
+DEF(before)
+DEF(the)
+DEF(program)
+DEF(can)
+DEF(do)
+DEF(its)
+DEF(work)
+DEF(efficiently)
+DEF(Without)
+DEF(enough)
+DEF(data)
+DEF(relocation)
+DEF(sections)
+DEF(are)
+DEF(not)
+DEF(sufficiently)
+DEF(large)
+DEF(and)
+DEF(injected)
+DEF(code)
+DEF(wouldnt)
+DEF(fit)
+DEF(Said)
+DEF(otherwise)
+DEF(we)
+DEF(need)
+DEF(more)
+DEF(words)
+DEF(than)
+DEF(up)
+DEF(here)
+DEF(so)
+DEF(that)
+DEF(relocations)
+DEF(take)
+DEF(significant)
+DEF(bytes)
+DEF(amounts)
+DEF(which)
+DEF(isnt)
+DEF(exactly)
+DEF(easily)
+DEF(achieved)
+DEF(like)
+DEF(this)
+DEF(Actually)
+DEF(I)
+DEF(must)
+DEF(cheat)
+DEF(by)
+DEF(including)
+DEF(these)
+DEF(phrases)
+DEF(several)
+DEF(times)
+
+#else
+#include <stdlib.h>
+#include <stdio.h>
+
+#define DEF(w) static const char str_ ## w[] = #w;
+#include "test.c"
+#undef DEF
+
+const char *strings[] = {
+#define DEF(w) str_ ## w,
+#include "test.c"
+#include "test.c"
+#include "test.c"
+};
+
+/* Create a hole between two zones of relative relocations */
+const int hole[] = {
+    42, 42, 42, 42
+};
+
+const char *strings2[] = {
+#include "test.c"
+#include "test.c"
+#include "test.c"
+#include "test.c"
+#include "test.c"
+#undef DEF
+};
+
+/* On ARM, this creates a .tbss section before .init_array, which
+ * elfhack could then pick instead of .init_array */
+__thread int foo;
+
+__attribute__((constructor)) void end_test() {
+    static int count = 0;
+    // Only exit when both constructors have been called
+    if (++count == 2) {
+        fprintf(stderr, "PASS\n");
+        exit(0);
+    }
+}
+
+__attribute__((constructor)) void test() {
+    int i = 0, j = 0;
+#define DEF_(a,i,w) \
+    if (a[i++] != str_ ## w) { \
+        fprintf(stderr, "FAIL\n"); \
+        exit(1); \
+    }
+#define DEF(w) DEF_(strings,i,w)
+#include "test.c"
+#include "test.c"
+#include "test.c"
+#undef DEF
+#define DEF(w) DEF_(strings2,j,w)
+#include "test.c"
+#include "test.c"
+#include "test.c"
+#include "test.c"
+#include "test.c"
+#undef DEF
+    if (i != sizeof(strings)/sizeof(strings[0]) &&
+        j != sizeof(strings2)/sizeof(strings2[0]))
+        fprintf(stderr, "WARNING: Test doesn't cover the whole array\n");
+    end_test();
+}
+
+#endif