202 строки
6.3 KiB
C
202 строки
6.3 KiB
C
/*
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* Copyright (C) 2014-2016 Linaro Ltd. <ard.biesheuvel@linaro.org>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*/
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#include <linux/elf.h>
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/sort.h>
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struct plt_entry {
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/*
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* A program that conforms to the AArch64 Procedure Call Standard
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* (AAPCS64) must assume that a veneer that alters IP0 (x16) and/or
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* IP1 (x17) may be inserted at any branch instruction that is
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* exposed to a relocation that supports long branches. Since that
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* is exactly what we are dealing with here, we are free to use x16
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* as a scratch register in the PLT veneers.
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*/
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__le32 mov0; /* movn x16, #0x.... */
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__le32 mov1; /* movk x16, #0x...., lsl #16 */
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__le32 mov2; /* movk x16, #0x...., lsl #32 */
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__le32 br; /* br x16 */
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};
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u64 module_emit_plt_entry(struct module *mod, const Elf64_Rela *rela,
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Elf64_Sym *sym)
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{
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struct plt_entry *plt = (struct plt_entry *)mod->arch.plt->sh_addr;
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int i = mod->arch.plt_num_entries;
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u64 val = sym->st_value + rela->r_addend;
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/*
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* We only emit PLT entries against undefined (SHN_UNDEF) symbols,
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* which are listed in the ELF symtab section, but without a type
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* or a size.
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* So, similar to how the module loader uses the Elf64_Sym::st_value
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* field to store the resolved addresses of undefined symbols, let's
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* borrow the Elf64_Sym::st_size field (whose value is never used by
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* the module loader, even for symbols that are defined) to record
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* the address of a symbol's associated PLT entry as we emit it for a
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* zero addend relocation (which is the only kind we have to deal with
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* in practice). This allows us to find duplicates without having to
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* go through the table every time.
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*/
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if (rela->r_addend == 0 && sym->st_size != 0) {
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BUG_ON(sym->st_size < (u64)plt || sym->st_size >= (u64)&plt[i]);
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return sym->st_size;
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}
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mod->arch.plt_num_entries++;
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BUG_ON(mod->arch.plt_num_entries > mod->arch.plt_max_entries);
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/*
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* MOVK/MOVN/MOVZ opcode:
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* +--------+------------+--------+-----------+-------------+---------+
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* | sf[31] | opc[30:29] | 100101 | hw[22:21] | imm16[20:5] | Rd[4:0] |
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* +--------+------------+--------+-----------+-------------+---------+
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*
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* Rd := 0x10 (x16)
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* hw := 0b00 (no shift), 0b01 (lsl #16), 0b10 (lsl #32)
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* opc := 0b11 (MOVK), 0b00 (MOVN), 0b10 (MOVZ)
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* sf := 1 (64-bit variant)
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*/
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plt[i] = (struct plt_entry){
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cpu_to_le32(0x92800010 | (((~val ) & 0xffff)) << 5),
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cpu_to_le32(0xf2a00010 | ((( val >> 16) & 0xffff)) << 5),
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cpu_to_le32(0xf2c00010 | ((( val >> 32) & 0xffff)) << 5),
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cpu_to_le32(0xd61f0200)
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};
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if (rela->r_addend == 0)
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sym->st_size = (u64)&plt[i];
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return (u64)&plt[i];
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}
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#define cmp_3way(a,b) ((a) < (b) ? -1 : (a) > (b))
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static int cmp_rela(const void *a, const void *b)
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{
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const Elf64_Rela *x = a, *y = b;
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int i;
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/* sort by type, symbol index and addend */
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i = cmp_3way(ELF64_R_TYPE(x->r_info), ELF64_R_TYPE(y->r_info));
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if (i == 0)
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i = cmp_3way(ELF64_R_SYM(x->r_info), ELF64_R_SYM(y->r_info));
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if (i == 0)
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i = cmp_3way(x->r_addend, y->r_addend);
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return i;
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}
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static bool duplicate_rel(const Elf64_Rela *rela, int num)
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{
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/*
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* Entries are sorted by type, symbol index and addend. That means
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* that, if a duplicate entry exists, it must be in the preceding
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* slot.
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*/
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return num > 0 && cmp_rela(rela + num, rela + num - 1) == 0;
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}
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static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num)
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{
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unsigned int ret = 0;
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Elf64_Sym *s;
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int i;
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for (i = 0; i < num; i++) {
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switch (ELF64_R_TYPE(rela[i].r_info)) {
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case R_AARCH64_JUMP26:
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case R_AARCH64_CALL26:
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/*
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* We only have to consider branch targets that resolve
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* to undefined symbols. This is not simply a heuristic,
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* it is a fundamental limitation, since the PLT itself
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* is part of the module, and needs to be within 128 MB
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* as well, so modules can never grow beyond that limit.
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*/
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s = syms + ELF64_R_SYM(rela[i].r_info);
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if (s->st_shndx != SHN_UNDEF)
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break;
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/*
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* Jump relocations with non-zero addends against
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* undefined symbols are supported by the ELF spec, but
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* do not occur in practice (e.g., 'jump n bytes past
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* the entry point of undefined function symbol f').
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* So we need to support them, but there is no need to
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* take them into consideration when trying to optimize
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* this code. So let's only check for duplicates when
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* the addend is zero: this allows us to record the PLT
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* entry address in the symbol table itself, rather than
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* having to search the list for duplicates each time we
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* emit one.
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*/
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if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
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ret++;
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break;
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}
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}
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return ret;
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}
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int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
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char *secstrings, struct module *mod)
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{
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unsigned long plt_max_entries = 0;
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Elf64_Sym *syms = NULL;
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int i;
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/*
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* Find the empty .plt section so we can expand it to store the PLT
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* entries. Record the symtab address as well.
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*/
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for (i = 0; i < ehdr->e_shnum; i++) {
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if (strcmp(".plt", secstrings + sechdrs[i].sh_name) == 0)
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mod->arch.plt = sechdrs + i;
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else if (sechdrs[i].sh_type == SHT_SYMTAB)
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syms = (Elf64_Sym *)sechdrs[i].sh_addr;
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}
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if (!mod->arch.plt) {
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pr_err("%s: module PLT section missing\n", mod->name);
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return -ENOEXEC;
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}
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if (!syms) {
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pr_err("%s: module symtab section missing\n", mod->name);
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return -ENOEXEC;
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}
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for (i = 0; i < ehdr->e_shnum; i++) {
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Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
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int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
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Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
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if (sechdrs[i].sh_type != SHT_RELA)
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continue;
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/* ignore relocations that operate on non-exec sections */
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if (!(dstsec->sh_flags & SHF_EXECINSTR))
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continue;
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/* sort by type, symbol index and addend */
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sort(rels, numrels, sizeof(Elf64_Rela), cmp_rela, NULL);
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plt_max_entries += count_plts(syms, rels, numrels);
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}
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mod->arch.plt->sh_type = SHT_NOBITS;
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mod->arch.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
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mod->arch.plt->sh_addralign = L1_CACHE_BYTES;
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mod->arch.plt->sh_size = plt_max_entries * sizeof(struct plt_entry);
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mod->arch.plt_num_entries = 0;
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mod->arch.plt_max_entries = plt_max_entries;
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return 0;
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}
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