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