You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
273 lines
7.0 KiB
273 lines
7.0 KiB
/*
|
|
* parse_vdso.c: Linux reference vDSO parser
|
|
* Written by Andrew Lutomirski, 2011-2014.
|
|
*
|
|
* This code is meant to be linked in to various programs that run on Linux.
|
|
* As such, it is available with as few restrictions as possible. This file
|
|
* is licensed under the Creative Commons Zero License, version 1.0,
|
|
* available at http://creativecommons.org/publicdomain/zero/1.0/legalcode
|
|
*
|
|
* The vDSO is a regular ELF DSO that the kernel maps into user space when
|
|
* it starts a program. It works equally well in statically and dynamically
|
|
* linked binaries.
|
|
*
|
|
* This code is tested on x86. In principle it should work on any
|
|
* architecture that has a vDSO.
|
|
*/
|
|
|
|
#include <stdbool.h>
|
|
#include <stdint.h>
|
|
#include <string.h>
|
|
#include <limits.h>
|
|
#include <elf.h>
|
|
|
|
/*
|
|
* To use this vDSO parser, first call one of the vdso_init_* functions.
|
|
* If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
|
|
* to vdso_init_from_sysinfo_ehdr. Otherwise pass auxv to vdso_init_from_auxv.
|
|
* Then call vdso_sym for each symbol you want. For example, to look up
|
|
* gettimeofday on x86_64, use:
|
|
*
|
|
* <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
|
|
* or
|
|
* <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
|
|
*
|
|
* vdso_sym will return 0 if the symbol doesn't exist or if the init function
|
|
* failed or was not called. vdso_sym is a little slow, so its return value
|
|
* should be cached.
|
|
*
|
|
* vdso_sym is threadsafe; the init functions are not.
|
|
*
|
|
* These are the prototypes:
|
|
*/
|
|
extern void vdso_init_from_auxv(void *auxv);
|
|
extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
|
|
extern void *vdso_sym(const char *version, const char *name);
|
|
|
|
|
|
/* And here's the code. */
|
|
#ifndef ELF_BITS
|
|
# if ULONG_MAX > 0xffffffffUL
|
|
# define ELF_BITS 64
|
|
# else
|
|
# define ELF_BITS 32
|
|
# endif
|
|
#endif
|
|
|
|
#define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
|
|
#define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
|
|
#define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
|
|
|
|
static struct vdso_info
|
|
{
|
|
bool valid;
|
|
|
|
/* Load information */
|
|
uintptr_t load_addr;
|
|
uintptr_t load_offset; /* load_addr - recorded vaddr */
|
|
|
|
/* Symbol table */
|
|
ELF(Sym) *symtab;
|
|
const char *symstrings;
|
|
ELF(Word) *bucket, *chain;
|
|
ELF(Word) nbucket, nchain;
|
|
|
|
/* Version table */
|
|
ELF(Versym) *versym;
|
|
ELF(Verdef) *verdef;
|
|
} vdso_info;
|
|
|
|
/* Straight from the ELF specification. */
|
|
static unsigned long elf_hash(const char *s_name)
|
|
{
|
|
unsigned long h = 0, g;
|
|
const unsigned char *name = (const unsigned char *)s_name;
|
|
|
|
while (*name)
|
|
{
|
|
h = (h << 4) + *name++;
|
|
g = h & 0xf0000000;
|
|
if (g)
|
|
h ^= g >> 24;
|
|
h &= ~g;
|
|
}
|
|
return h;
|
|
}
|
|
|
|
void vdso_init_from_sysinfo_ehdr(uintptr_t base)
|
|
{
|
|
size_t i;
|
|
bool found_vaddr = false;
|
|
|
|
vdso_info.valid = false;
|
|
|
|
vdso_info.load_addr = base;
|
|
|
|
ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
|
|
if (hdr->e_ident[EI_CLASS] !=
|
|
(ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
|
|
return; /* Wrong ELF class -- check ELF_BITS */
|
|
}
|
|
|
|
ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
|
|
ELF(Dyn) *dyn = 0;
|
|
|
|
/*
|
|
* We need two things from the segment table: the load offset
|
|
* and the dynamic table.
|
|
*/
|
|
for (i = 0; i < hdr->e_phnum; i++)
|
|
{
|
|
if (pt[i].p_type == PT_LOAD && !found_vaddr) {
|
|
found_vaddr = true;
|
|
vdso_info.load_offset = base
|
|
+ (uintptr_t)pt[i].p_offset
|
|
- (uintptr_t)pt[i].p_vaddr;
|
|
} else if (pt[i].p_type == PT_DYNAMIC) {
|
|
dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
|
|
}
|
|
}
|
|
|
|
if (!found_vaddr || !dyn)
|
|
return; /* Failed */
|
|
|
|
/*
|
|
* Fish out the useful bits of the dynamic table.
|
|
*/
|
|
ELF(Word) *hash = 0;
|
|
vdso_info.symstrings = 0;
|
|
vdso_info.symtab = 0;
|
|
vdso_info.versym = 0;
|
|
vdso_info.verdef = 0;
|
|
for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
|
|
switch (dyn[i].d_tag) {
|
|
case DT_STRTAB:
|
|
vdso_info.symstrings = (const char *)
|
|
((uintptr_t)dyn[i].d_un.d_ptr
|
|
+ vdso_info.load_offset);
|
|
break;
|
|
case DT_SYMTAB:
|
|
vdso_info.symtab = (ELF(Sym) *)
|
|
((uintptr_t)dyn[i].d_un.d_ptr
|
|
+ vdso_info.load_offset);
|
|
break;
|
|
case DT_HASH:
|
|
hash = (ELF(Word) *)
|
|
((uintptr_t)dyn[i].d_un.d_ptr
|
|
+ vdso_info.load_offset);
|
|
break;
|
|
case DT_VERSYM:
|
|
vdso_info.versym = (ELF(Versym) *)
|
|
((uintptr_t)dyn[i].d_un.d_ptr
|
|
+ vdso_info.load_offset);
|
|
break;
|
|
case DT_VERDEF:
|
|
vdso_info.verdef = (ELF(Verdef) *)
|
|
((uintptr_t)dyn[i].d_un.d_ptr
|
|
+ vdso_info.load_offset);
|
|
break;
|
|
}
|
|
}
|
|
if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
|
|
return; /* Failed */
|
|
|
|
if (!vdso_info.verdef)
|
|
vdso_info.versym = 0;
|
|
|
|
/* Parse the hash table header. */
|
|
vdso_info.nbucket = hash[0];
|
|
vdso_info.nchain = hash[1];
|
|
vdso_info.bucket = &hash[2];
|
|
vdso_info.chain = &hash[vdso_info.nbucket + 2];
|
|
|
|
/* That's all we need. */
|
|
vdso_info.valid = true;
|
|
}
|
|
|
|
static bool vdso_match_version(ELF(Versym) ver,
|
|
const char *name, ELF(Word) hash)
|
|
{
|
|
/*
|
|
* This is a helper function to check if the version indexed by
|
|
* ver matches name (which hashes to hash).
|
|
*
|
|
* The version definition table is a mess, and I don't know how
|
|
* to do this in better than linear time without allocating memory
|
|
* to build an index. I also don't know why the table has
|
|
* variable size entries in the first place.
|
|
*
|
|
* For added fun, I can't find a comprehensible specification of how
|
|
* to parse all the weird flags in the table.
|
|
*
|
|
* So I just parse the whole table every time.
|
|
*/
|
|
|
|
/* First step: find the version definition */
|
|
ver &= 0x7fff; /* Apparently bit 15 means "hidden" */
|
|
ELF(Verdef) *def = vdso_info.verdef;
|
|
while(true) {
|
|
if ((def->vd_flags & VER_FLG_BASE) == 0
|
|
&& (def->vd_ndx & 0x7fff) == ver)
|
|
break;
|
|
|
|
if (def->vd_next == 0)
|
|
return false; /* No definition. */
|
|
|
|
def = (ELF(Verdef) *)((char *)def + def->vd_next);
|
|
}
|
|
|
|
/* Now figure out whether it matches. */
|
|
ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
|
|
return def->vd_hash == hash
|
|
&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
|
|
}
|
|
|
|
void *vdso_sym(const char *version, const char *name)
|
|
{
|
|
unsigned long ver_hash;
|
|
if (!vdso_info.valid)
|
|
return 0;
|
|
|
|
ver_hash = elf_hash(version);
|
|
ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
|
|
|
|
for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
|
|
ELF(Sym) *sym = &vdso_info.symtab[chain];
|
|
|
|
/* Check for a defined global or weak function w/ right name. */
|
|
if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
|
|
continue;
|
|
if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
|
|
ELF64_ST_BIND(sym->st_info) != STB_WEAK)
|
|
continue;
|
|
if (sym->st_shndx == SHN_UNDEF)
|
|
continue;
|
|
if (strcmp(name, vdso_info.symstrings + sym->st_name))
|
|
continue;
|
|
|
|
/* Check symbol version. */
|
|
if (vdso_info.versym
|
|
&& !vdso_match_version(vdso_info.versym[chain],
|
|
version, ver_hash))
|
|
continue;
|
|
|
|
return (void *)(vdso_info.load_offset + sym->st_value);
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
|
|
void vdso_init_from_auxv(void *auxv)
|
|
{
|
|
ELF(auxv_t) *elf_auxv = auxv;
|
|
for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
|
|
{
|
|
if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
|
|
vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
|
|
return;
|
|
}
|
|
}
|
|
|
|
vdso_info.valid = false;
|
|
}
|