Bug 725284 - Preserve PT_LOAD alignment, except when it's the default on x86-64. r=tglek

build/unix/elfhack/Makefile.in

@@ -95,7 +95,7 @@ test$(DLL_SUFFIX): test.$(OBJ_SUFFIX) elfhack $(CSRCS:.c=.$(OBJ_SUFFIX))
 	@echo === --disable-elf-hack until this is fixed.
 	@echo ===
 	@rm -f $@.bak
-	$(CURDIR)/elfhack -b $@
+	$(CURDIR)/elfhack -b -f $@
 	# Fail if the backup file doesn't exist
 	[ -f "$@.bak" ]
 	# Fail if the new library doesn't contain less relocations

build/unix/elfhack/elf.cpp

@@ -265,6 +265,15 @@ Elf::Elf(std::ifstream &file)
     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]);
+        if (phdr.p_type == PT_LOAD) {
+            // Default alignment for PT_LOAD on x86-64 prevents elfhack from
+            // doing anything useful. However, the system doesn't actually
+            // require such a big alignment, so in order for elfhack to work
+            // efficiently, reduce alignment when it's originally the default
+            // one.
+            if ((ehdr->e_machine == EM_X86_64) && (phdr.p_align == 0x200000))
+              phdr.p_align = 0x1000;
+        }
         ElfSegment *segment = new ElfSegment(&phdr);
         // Some segments aren't entirely filled (if at all) by sections
         // For those, we use fake sections
@@ -503,12 +512,17 @@ unsigned int ElfSection::getOffset()
     if (previous->getType() != SHT_NOBITS)
         offset += previous->getSize();
 
+    Elf32_Word align = 0x1000;
+    for (std::vector<ElfSegment *>::iterator seg = segments.begin(); seg != segments.end(); seg++)
+        align = std::max(align, (*seg)->getAlign());
+
+    Elf32_Word mask = align - 1;
     // 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;
+        if ((getAddr() & mask) < (offset & mask))
+            offset = (offset | mask) + (getAddr() & mask) + 1;
         else
-            offset = (offset & ~4095) + (getAddr() & 4095);
+            offset = (offset & ~mask) + (getAddr() & mask);
     }
     if ((getType() != SHT_NOBITS) && (offset & (getAddrAlign() - 1)))
         offset = (offset | (getAddrAlign() - 1)) + 1;
@@ -632,7 +646,7 @@ ElfSegment *ElfSegment::splitBefore(ElfSection *section)
     phdr.p_vaddr = 0;
     phdr.p_paddr = phdr.p_vaddr + v_p_diff;
     phdr.p_flags = flags;
-    phdr.p_align = 0x1000;
+    phdr.p_align = getAlign();
     phdr.p_filesz = (unsigned int)-1;
     phdr.p_memsz = (unsigned int)-1;
     ElfSegment *segment = new ElfSegment(&phdr);

build/unix/elfhack/elfhack.cpp

@@ -501,7 +501,7 @@ static inline int backup_file(const char *name)
     return rename(name, fname.c_str());
 }
 
-void do_file(const char *name, bool backup = false)
+void do_file(const char *name, bool backup = false, bool force = false)
 {
     std::ifstream file(name, std::ios::in|std::ios::binary);
     Elf *elf = new Elf(file);
@@ -531,7 +531,7 @@ void do_file(const char *name, bool backup = false)
         break;
     }
     if (exit == 0) {
-        if (elf->getSize() >= size) {
+        if (!force && (elf->getSize() >= size)) {
             fprintf(stderr, "No gain. Skipping\n");
         } else if (backup && backup_file(name) != 0) {
             fprintf(stderr, "Couln't create backup file\n");
@@ -548,14 +548,17 @@ int main(int argc, char *argv[])
 {
     int arg;
     bool backup = false;
+    bool force = 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)
+        if (strcmp(argv[arg], "-f") == 0)
+            force = true;
+        else if (strcmp(argv[arg], "-b") == 0)
             backup = true;
         else
-            do_file(argv[arg], backup);
+            do_file(argv[arg], backup, force);
     }
 
     free(rundir);

build/unix/elfhack/elfxx.h

@@ -444,7 +444,7 @@ public:
 
     unsigned int getType() { return type; }
     unsigned int getFlags() { return flags; }
-    unsigned int getAlign() { return type == PT_LOAD ? 0x1000 : align; /* TODO: remove this gross hack */ }
+    unsigned int getAlign() { return align; }
 
     ElfSection *getFirstSection() { return sections.empty() ? NULL : sections.front(); }
     int getVPDiff() { return v_p_diff; }