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#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <libelfu/libelfu.h>
static void* subFindByName(ElfuElf *me, ElfuScn *ms, void *aux1, void *aux2)
{
char *name = (char*)aux1;
(void)aux2;
if (elfu_mScnName(me, ms)) {
if (!strcmp(elfu_mScnName(me, ms), name)) {
return ms;
}
}
/* Continue */
return NULL;
}
/* Hazard a guess where a function may be found in the PLT */
static GElf_Word pltLookupVal(ElfuElf *metarget, char *name)
{
ElfuScn *relplt;
ElfuScn *plt;
ElfuRel *rel;
GElf_Word j;
relplt = elfu_mScnForall(metarget, subFindByName, ".rel.plt", NULL);
if (!relplt) {
ELFU_WARN("dynsymLookupVal: Could not find .rel.plt section in destination ELF.\n");
return 0;
}
plt = elfu_mScnForall(metarget, subFindByName, ".plt", NULL);
if (!plt) {
ELFU_WARN("dynsymLookupVal: Could not find .plt section in destination ELF.\n");
return 0;
}
/* Look up name. If the j-th entry in .rel.plt has the name we are
* looking for, we assume that the (j+1)-th entry in .plt is machine
* code to jump to the function.
* Your mileage may vary, but it works on my GNU binaries. */
assert(relplt->linkptr);
j = 0;
CIRCLEQ_FOREACH(rel, &relplt->reltab.rels, elem) {
GElf_Word i;
ElfuSym *sym;
char *symname;
j++;
/* We only consider runtime relocations for functions.
* Technically, these relocations write the functions' addresses
* to the GOT, not the PLT, after the dynamic linker has found
* them. */
if ((metarget->elfclass == ELFCLASS32 && rel->type != R_386_JMP_SLOT)
|| (metarget->elfclass == ELFCLASS64 && rel->type != R_X86_64_JUMP_SLOT)) {
continue;
}
/* Get the (rel->sym)-th symbol from the symbol table that
* .rel.plt points to. */
sym = CIRCLEQ_FIRST(&relplt->linkptr->symtab.syms);
for (i = 1; i < rel->sym; i++) {
sym = CIRCLEQ_NEXT(sym, elem);
}
symname = ELFU_SYMSTR(relplt->linkptr, sym->name);
if (!strcmp(symname, name)) {
/* If this is the symbol we are looking for, then in an x86 binary
* the jump to the dynamic symbol is probably at offset (j * 16)
* from the start of the PLT, where j is the PLT entry and 16 is
* the number of bytes the machine code in a PLT entry take. */
GElf_Addr addr = plt->shdr.sh_addr + (16 * j);
ELFU_DEBUG("dynsymLookupVal: Guessing symbol '%s' is in destination memory at %jx (PLT entry #%d).\n", name, addr, j);
return addr;
}
}
ELFU_WARN("dynsymLookupVal: Could not find symbol '%s' in destination ELF.\n", name);
return 0;
}
static GElf_Word symtabLookupVal(ElfuElf *metarget, ElfuScn *msst, GElf_Word entry)
{
GElf_Word i;
ElfuSym *sym;
char *symname;
assert(metarget);
assert(msst);
assert(entry > 0);
assert(!CIRCLEQ_EMPTY(&msst->symtab.syms));
sym = CIRCLEQ_FIRST(&msst->symtab.syms);
for (i = 1; i < entry; i++) {
sym = CIRCLEQ_NEXT(sym, elem);
}
symname = ELFU_SYMSTR(msst, sym->name);
switch (sym->type) {
case STT_NOTYPE:
case STT_OBJECT:
case STT_FUNC:
if (sym->scnptr) {
ElfuScn *newscn = elfu_mScnByOldscn(metarget, sym->scnptr);
assert(newscn);
return newscn->shdr.sh_addr + sym->value;
} else if (sym->shndx == SHN_UNDEF) {
/* Look the symbol up in .rel.plt. If it cannot be found there then
* .rel.dyn may need to be expanded with a COPY relocation so the
* dynamic linker fixes up the (TODO). */
return pltLookupVal(metarget, symname);
} else if (sym->shndx == SHN_ABS) {
return sym->value;
} else {
ELFU_WARN("symtabLookupVal: Symbol binding COMMON is not supported, using 0.\n");
return 0;
}
break;
case STT_SECTION:
assert(sym->scnptr);
assert(elfu_mScnByOldscn(metarget, sym->scnptr));
return elfu_mScnByOldscn(metarget, sym->scnptr)->shdr.sh_addr;
case STT_FILE:
ELFU_WARN("symtabLookupVal: Symbol type FILE is not supported, using 0.\n");
return 0;
default:
ELFU_WARN("symtabLookupVal: Unknown symbol type %d for %s.\n", sym->type, symname);
return 0;
}
}
void elfu_mRelocate32(ElfuElf *metarget, ElfuScn *mstarget, ElfuScn *msrt)
{
ElfuRel *rel;
assert(mstarget);
assert(msrt);
ELFU_DEBUG("Relocating in section of type %d size %jx\n",
mstarget->shdr.sh_type,
mstarget->shdr.sh_size);
CIRCLEQ_FOREACH(rel, &msrt->reltab.rels, elem) {
Elf32_Word *dest32 = (Elf32_Word*)(((char*)(mstarget->data.d_buf)) + rel->offset);
Elf64_Word *dest64 = (Elf64_Word*)(((char*)(mstarget->data.d_buf)) + rel->offset);
if (metarget->elfclass == ELFCLASS32) {
Elf32_Word a32 = rel->addendUsed ? rel->addend : *dest32;
Elf32_Addr p32 = mstarget->shdr.sh_addr + rel->offset;
Elf32_Addr s32 = symtabLookupVal(metarget, msrt->linkptr, rel->sym);
switch(rel->type) {
case R_386_NONE:
ELFU_DEBUG("Skipping relocation: R_386_NONE\n");
break;
case R_386_32:
*dest32 = s32 + a32;
break;
case R_386_PC32:
*dest32 = s32 + a32 - p32;
break;
default:
ELFU_DEBUG("Skipping relocation: Unknown type %d\n", rel->type);
}
} else if (metarget->elfclass == ELFCLASS64) {
/* x86-64 only uses RELA with explicit addend. */
assert(rel->addendUsed);
Elf64_Word a64 = rel->addend;
Elf64_Addr p64 = mstarget->shdr.sh_addr + rel->offset;
Elf64_Addr s64 = symtabLookupVal(metarget, msrt->linkptr, rel->sym);
switch(rel->type) {
case R_X86_64_NONE:
ELFU_DEBUG("Skipping relocation: R_386_NONE\n");
break;
case R_X86_64_64:
*dest64 = s64 + a64;
break;
case R_X86_64_PC32:
*dest32 = s64 + a64 - p64;
break;
case R_X86_64_32:
*dest32 = s64 + a64;
break;
default:
ELFU_DEBUG("Skipping relocation: Unknown type %d", rel->type);
}
}
}
}
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