/* We use this macro to refer to ELF types independent of the native wordsize.
`ElfW(TYPE)' is used in place of `Elf32_TYPE' or `Elf64_TYPE'. */
#define ElfW(type) _ElfW(Elf, __ELF_NATIVE_CLASS, type)
#define _ElfW(e, w, t) _ElfW_1(e, w, _##t)
#define _ElfW_1(e, w, t) e##w##t
/* All references to the value of l_info[DT_PLTGOT],
l_info[DT_STRTAB], l_info[DT_SYMTAB], l_info[DT_RELA],
l_info[DT_REL], l_info[DT_JMPREL], and l_info[VERSYMIDX (DT_VERSYM)]
have to be accessed via the D_PTR macro. The macro is needed since for
most architectures the entry is already relocated - but for some not
and we need to relocate at access time. */
#ifdef DL_RO_DYN_SECTION
#define D_PTR(map, i) ((map)->i->d_un.d_ptr + (map)->l_addr)
#else
#define D_PTR(map, i) (map)->i->d_un.d_ptr
#endif
#if (!ELF_MACHINE_NO_RELA && !defined ELF_MACHINE_PLT_REL) || ELF_MACHINE_NO_REL
#define PLTREL ElfW(Rela)
#else
#define PLTREL ElfW(Rel)
#endif
/* The type of the return value of fixup/profile_fixup. */
#define DL_FIXUP_VALUE_TYPE ElfW(Addr)
/* Construct a value of type DL_FIXUP_VALUE_TYPE from a code address
and a link map. */
#define DL_FIXUP_MAKE_VALUE(map, addr) (addr)
/* Extract the code address from a value of type DL_FIXUP_MAKE_VALUE.
*/
#define DL_FIXUP_VALUE_CODE_ADDR(value) (value)
#define DL_FIXUP_VALUE_ADDR(value) (value)
#define DL_FIXUP_ADDR_VALUE(addr) (addr)
/* Result of the lookup functions and how to retrieve the base address. */
typedef struct link_map *lookup_t;
#define LOOKUP_VALUE(map) map
#define LOOKUP_VALUE_ADDRESS(map, set) ((set) || (map) ? (map)->l_addr : 0)
/* Calculate the address of symbol REF using the base address from map MAP,
if non-NULL. Don't check for NULL map if MAP_SET is TRUE. */
#define SYMBOL_ADDRESS(map, ref, map_set) \
((ref) == NULL ? 0 \
: (__glibc_unlikely((ref)->st_shndx == SHN_ABS) ? 0 \
: LOOKUP_VALUE_ADDRESS(map, map_set)) + \
(ref)->st_value)
#ifndef reloc_offset
#define reloc_offset reloc_arg
#define reloc_index reloc_arg / sizeof(PLTREL)
#endif
/* This function is called through a special trampoline from the PLT the
first time each PLT entry is called. We must perform the relocation
specified in the PLT of the given shared object, and return the resolved
function address to the trampoline, which will restart the original call
to that address. Future calls will bounce directly from the PLT to the
function. */
DL_FIXUP_VALUE_TYPE
attribute_hidden __attribute((noinline)) ARCH_FIXUP_ATTRIBUTE
_dl_fixup(
#ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
ELF_MACHINE_RUNTIME_FIXUP_ARGS,
#endif
struct link_map *l, ElfW(Word) reloc_arg)
{
// 获取目标文件的动态符号表
const ElfW(Sym) *const symtab = (const void *)D_PTR(l, l_info[DT_SYMTAB]);
// 获取目标文件的动态字符串表
const char *strtab = (const void *)D_PTR(l, l_info[DT_STRTAB]);
// 根据给定的偏移获取待重定位的表项
const PLTREL *const reloc = (const void *)(D_PTR(l, l_info[DT_JMPREL]) + reloc_offset);
// 依据得到的重定位表项获取待解析符号的符号信息
const ElfW(Sym) *sym = &symtab[ELFW(R_SYM)(reloc->r_info)];
const ElfW(Sym) *refsym = sym;
// rel_addr 中最终存储着要解析的符号的地址
// 即 *rel_addr = addr_of_symbol_to_be_resolved
void *const rel_addr = (void *)(l->l_addr + reloc->r_offset);
lookup_t result;
DL_FIXUP_VALUE_TYPE value;
/* Sanity check that we're really looking at a PLT relocation. */
// 确保待重定位表项的类型为 ELF_MACHINE_JMP_SLOT
assert(ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
/* Look up the target symbol. If the normal lookup rules are not
used don't look in the global scope. */
// 判断符号的可见性
if (__builtin_expect(ELFW(ST_VISIBILITY)(sym->st_other), 0) == 0)
{
// 获取符号的版本信息
const struct r_found_version *version = NULL;
if (l->l_info[VERSYMIDX(DT_VERSYM)] != NULL)
{
const ElfW(Half) *vernum = (const void *)D_PTR(l, l_info[VERSYMIDX(DT_VERSYM)]);
ElfW(Half) ndx = vernum[ELFW(R_SYM)(reloc->r_info)] & 0x7fff;
version = &l->l_versions[ndx];
if (version->hash == 0)
version = NULL;
}
/* We need to keep the scope around so do some locking. This is
not necessary for objects which cannot be unloaded or when
we are not using any threads (yet). */
int flags = DL_LOOKUP_ADD_DEPENDENCY;
if (!RTLD_SINGLE_THREAD_P)
{
THREAD_GSCOPE_SET_FLAG();
flags |= DL_LOOKUP_GSCOPE_LOCK;
}
#ifdef RTLD_ENABLE_FOREIGN_CALL
RTLD_ENABLE_FOREIGN_CALL;
#endif
// 查询待解析符号所在的目标文件的 link_map
result = _dl_lookup_symbol_x(strtab + sym->st_name, l, &sym, l->l_scope,
version, ELF_RTYPE_CLASS_PLT, flags, NULL);
/* We are done with the global scope. */
if (!RTLD_SINGLE_THREAD_P)
THREAD_GSCOPE_RESET_FLAG();
#ifdef RTLD_FINALIZE_FOREIGN_CALL
RTLD_FINALIZE_FOREIGN_CALL;
#endif
/* Currently result contains the base load address (or link map)
of the object that defines sym. Now add in the symbol
offset. */
// 基于查询到的 link_map 计算符号的绝对地址: result->l_addr + sym->st_value
// l_addr 为待解析函数所在文件的基地址
value = DL_FIXUP_MAKE_VALUE(result,
SYMBOL_ADDRESS(result, sym, false));
}
else
{
/* We already found the symbol. The module (and therefore its load
address) is also known. */
value = DL_FIXUP_MAKE_VALUE(l, SYMBOL_ADDRESS(l, sym, true));
result = l;
}
/* And now perhaps the relocation addend. */
value = elf_machine_plt_value(l, reloc, value);
if (sym != NULL && __builtin_expect(ELFW(ST_TYPE)(sym->st_info) == STT_GNU_IFUNC, 0))
value = elf_ifunc_invoke(DL_FIXUP_VALUE_ADDR(value));
/* Finally, fix up the plt itself. */
// 修复 plt 表
if (__glibc_unlikely(GLRO(dl_bind_not)))
return value;
return elf_machine_fixup_plt(l, result, refsym, sym, reloc, rel_addr, value);
}
static inline ElfW(Addr)
elf_machine_fixup_plt (struct link_map *map, lookup_t t,
const ElfW(Sym) *refsym, const ElfW(Sym) *sym,
const ElfW(Rela) *reloc,
ElfW(Addr) *reloc_addr, ElfW(Addr) value)
{
return *reloc_addr = value;
}
/* Return the final value of a PLT relocation. On x86-64 the
JUMP_SLOT relocation ignores the addend. */
static inline ElfW(Addr)
elf_machine_plt_value (struct link_map *map, const ElfW(Rela) *reloc,
ElfW(Addr) value)
{
return value;
}
