/*
* Copyright (C) 2014 Linaro Ltd. <ard.biesheuvel@linaro.org>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#include <linux/elf.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/sort.h>
#include <asm/cache.h>
#include <asm/opcodes.h>
#define PLT_ENT_STRIDE L1_CACHE_BYTES
#define PLT_ENT_COUNT (PLT_ENT_STRIDE / sizeof(u32))
#define PLT_ENT_SIZE (sizeof(struct plt_entries) / PLT_ENT_COUNT)
#ifdef CONFIG_THUMB2_KERNEL
#define PLT_ENT_LDR __opcode_to_mem_thumb32(0xf8dff000 | \
(PLT_ENT_STRIDE - 4))
#else
#define PLT_ENT_LDR __opcode_to_mem_arm(0xe59ff000 | \
(PLT_ENT_STRIDE - 8))
#endif
struct plt_entries {
u32 ldr[PLT_ENT_COUNT];
u32 lit[PLT_ENT_COUNT];
};
u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val)
{
struct plt_entries *plt = (struct plt_entries *)mod->arch.plt->sh_addr;
int idx = 0;
/*
* Look for an existing entry pointing to 'val'. Given that the
* relocations are sorted, this will be the last entry we allocated.
* (if one exists).
*/
if (mod->arch.plt_count > 0) {
plt += (mod->arch.plt_count - 1) / PLT_ENT_COUNT;
idx = (mod->arch.plt_count - 1) % PLT_ENT_COUNT;
if (plt->lit[idx] == val)
return (u32)&plt->ldr[idx];
idx = (idx + 1) % PLT_ENT_COUNT;
if (!idx)
plt++;
}
mod->arch.plt_count++;
BUG_ON(mod->arch.plt_count * PLT_ENT_SIZE > mod->arch.plt->sh_size);
if (!idx)
/* Populate a new set of entries */
*plt = (struct plt_entries){
{ [0 ... PLT_ENT_COUNT - 1] = PLT_ENT_LDR, },
{ val, }
};
else
plt->lit[idx] = val;
return (u32)&plt->ldr[idx];
}
#define cmp_3way(a,b) ((a) < (b) ? -1 : (a) > (b))
static int cmp_rel(const void *a, const void *b)
{
const Elf32_Rel *x = a, *y = b;
int i;
/* sort by type and symbol index */
i = cmp_3way(ELF32_R_TYPE(x->r_info), ELF32_R_TYPE(y->r_info));
if (i == 0)
i = cmp_3way(ELF32_R_SYM(x->r_info), ELF32_R_SYM(y->r_info));
return i;
}
static bool is_zero_addend_relocation(Elf32_Addr base, const Elf32_Rel *rel)
{
u32 *tval = (u32 *)(base + rel->r_offset);
/*
* Do a bitwise compare on the raw addend rather than fully decoding
* the offset and doing an arithmetic comparison.
* Note that a zero-addend jump/call relocation is encoded taking the
* PC bias into account, i.e., -8 for ARM and -4 for Thumb2.
*/
switch (ELF32_R_TYPE(rel->r_info)) {
u16 upper, lower;
case R_ARM_THM_CALL:
case R_ARM_THM_JUMP24:
upper = __mem_to_opcode_thumb16(((u16 *)tval)[0]);
lower = __mem_to_opcode_thumb16(((u16 *)tval)[1]);
return (upper & 0x7ff) == 0x7ff && (lower & 0x2fff) == 0x2ffe;
case R_ARM_CALL:
case R_ARM_PC24:
case R_ARM_JUMP24:
return (__mem_to_opcode_arm(*tval) & 0xffffff) == 0xfffffe;
}
BUG();
}
static bool duplicate_rel(Elf32_Addr base, const Elf32_Rel *rel, int num)
{
const Elf32_Rel *prev;
/*
* Entries are sorted by type and symbol index. That means that,
* if a duplicate entry exists, it must be in the preceding
* slot.
*/
if (!num)
return false;
prev = rel + num - 1;
return cmp_rel(rel + num, prev) == 0 &&
is_zero_addend_relocation(base, prev);
}
/* Count how many PLT entries we may need */
static unsigned int count_plts(const Elf32_Sym *syms, Elf32_Addr base,
const Elf32_Rel *rel, int num)
{
unsigned int ret = 0;
const Elf32_Sym *s;
int i;
for (i = 0; i < num; i++) {
switch (ELF32_R_TYPE(rel[i].r_info)) {
case R_ARM_CALL:
case R_ARM_PC24:
case R_ARM_JUMP24:
case R_ARM_THM_CALL:
case R_ARM_THM_JUMP24:
/*
* We only have to consider branch targets that resolve
* to undefined symbols. This is not simply a heuristic,
* it is a fundamental limitation, since the PLT itself
* is part of the module, and needs to be within range
* as well, so modules can never grow beyond that limit.
*/
s = syms + ELF32_R_SYM(rel[i].r_info);
if (s->st_shndx != SHN_UNDEF)
break;
/*
* Jump relocations with non-zero addends against
* undefined symbols are supported by the ELF spec, but
* do not occur in practice (e.g., 'jump n bytes past
* the entry point of undefined function symbol f').
* So we need to support them, but there is no need to
* take them into consideration when trying to optimize
* this code. So let's only check for duplicates when
* the addend is zero.
*/
if (!is_zero_addend_relocation(base, rel + i) ||
!duplicate_rel(base, rel, i))
ret++;
}
}
return ret;
}
int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
char *secstrings, struct module *mod)
{
unsigned long plts = 0;
Elf32_Shdr *s, *sechdrs_end = sechdrs + ehdr->e_shnum;
Elf32_Sym *syms = NULL;
/*
* To store the PLTs, we expand the .text section for core module code
* and for initialization code.
*/
for (s = sechdrs; s < sechdrs_end; ++s) {
if (strcmp(".plt", secstrings + s->sh_name) == 0)
mod->arch.plt = s;
else if (s->sh_type == SHT_SYMTAB)
syms = (Elf32_Sym *)s->sh_addr;
}
if (!mod->arch.plt) {
pr_err("%s: module PLT section missing\n", mod->name);
return -ENOEXEC;
}
if (!syms) {
pr_err("%s: module symtab section missing\n", mod->name);
return -ENOEXEC;
}
for (s = sechdrs + 1; s < sechdrs_end; ++s) {
Elf32_Rel *rels = (void *)ehdr + s->sh_offset;
int numrels = s->sh_size / sizeof(Elf32_Rel);
Elf32_Shdr *dstsec = sechdrs + s->sh_info;
if (s->sh_type != SHT_REL)
continue;
/* ignore relocations that operate on non-exec sections */
if (!(dstsec->sh_flags & SHF_EXECINSTR))
continue;
/* sort by type and symbol index */
sort(rels, numrels, sizeof(Elf32_Rel), cmp_rel, NULL);
plts += count_plts(syms, dstsec->sh_addr, rels, numrels);
}
mod->arch.plt->sh_type = SHT_NOBITS;
mod->arch.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
mod->arch.plt->sh_addralign = L1_CACHE_BYTES;
mod->arch.plt->sh_size = round_up(plts * PLT_ENT_SIZE,
sizeof(struct plt_entries));
mod->arch.plt_count = 0;
pr_debug("%s: plt=%x\n", __func__, mod->arch.plt->sh_size);
return 0;
}
