linux/Documentation/vDSO/parse_vdso.c
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   1/*
   2 * parse_vdso.c: Linux reference vDSO parser
   3 * Written by Andrew Lutomirski, 2011.
   4 *
   5 * This code is meant to be linked in to various programs that run on Linux.
   6 * As such, it is available with as few restrictions as possible.  This file
   7 * is licensed under the Creative Commons Zero License, version 1.0,
   8 * available at http://creativecommons.org/publicdomain/zero/1.0/legalcode
   9 *
  10 * The vDSO is a regular ELF DSO that the kernel maps into user space when
  11 * it starts a program.  It works equally well in statically and dynamically
  12 * linked binaries.
  13 *
  14 * This code is tested on x86_64.  In principle it should work on any 64-bit
  15 * architecture that has a vDSO.
  16 */
  17
  18#include <stdbool.h>
  19#include <stdint.h>
  20#include <string.h>
  21#include <elf.h>
  22
  23/*
  24 * To use this vDSO parser, first call one of the vdso_init_* functions.
  25 * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
  26 * to vdso_init_from_sysinfo_ehdr.  Otherwise pass auxv to vdso_init_from_auxv.
  27 * Then call vdso_sym for each symbol you want.  For example, to look up
  28 * gettimeofday on x86_64, use:
  29 *
  30 *     <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
  31 * or
  32 *     <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
  33 *
  34 * vdso_sym will return 0 if the symbol doesn't exist or if the init function
  35 * failed or was not called.  vdso_sym is a little slow, so its return value
  36 * should be cached.
  37 *
  38 * vdso_sym is threadsafe; the init functions are not.
  39 *
  40 * These are the prototypes:
  41 */
  42extern void vdso_init_from_auxv(void *auxv);
  43extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
  44extern void *vdso_sym(const char *version, const char *name);
  45
  46
  47/* And here's the code. */
  48
  49#ifndef __x86_64__
  50# error Not yet ported to non-x86_64 architectures
  51#endif
  52
  53static struct vdso_info
  54{
  55        bool valid;
  56
  57        /* Load information */
  58        uintptr_t load_addr;
  59        uintptr_t load_offset;  /* load_addr - recorded vaddr */
  60
  61        /* Symbol table */
  62        Elf64_Sym *symtab;
  63        const char *symstrings;
  64        Elf64_Word *bucket, *chain;
  65        Elf64_Word nbucket, nchain;
  66
  67        /* Version table */
  68        Elf64_Versym *versym;
  69        Elf64_Verdef *verdef;
  70} vdso_info;
  71
  72/* Straight from the ELF specification. */
  73static unsigned long elf_hash(const unsigned char *name)
  74{
  75        unsigned long h = 0, g;
  76        while (*name)
  77        {
  78                h = (h << 4) + *name++;
  79                if (g = h & 0xf0000000)
  80                        h ^= g >> 24;
  81                h &= ~g;
  82        }
  83        return h;
  84}
  85
  86void vdso_init_from_sysinfo_ehdr(uintptr_t base)
  87{
  88        size_t i;
  89        bool found_vaddr = false;
  90
  91        vdso_info.valid = false;
  92
  93        vdso_info.load_addr = base;
  94
  95        Elf64_Ehdr *hdr = (Elf64_Ehdr*)base;
  96        Elf64_Phdr *pt = (Elf64_Phdr*)(vdso_info.load_addr + hdr->e_phoff);
  97        Elf64_Dyn *dyn = 0;
  98
  99        /*
 100         * We need two things from the segment table: the load offset
 101         * and the dynamic table.
 102         */
 103        for (i = 0; i < hdr->e_phnum; i++)
 104        {
 105                if (pt[i].p_type == PT_LOAD && !found_vaddr) {
 106                        found_vaddr = true;
 107                        vdso_info.load_offset = base
 108                                + (uintptr_t)pt[i].p_offset
 109                                - (uintptr_t)pt[i].p_vaddr;
 110                } else if (pt[i].p_type == PT_DYNAMIC) {
 111                        dyn = (Elf64_Dyn*)(base + pt[i].p_offset);
 112                }
 113        }
 114
 115        if (!found_vaddr || !dyn)
 116                return;  /* Failed */
 117
 118        /*
 119         * Fish out the useful bits of the dynamic table.
 120         */
 121        Elf64_Word *hash = 0;
 122        vdso_info.symstrings = 0;
 123        vdso_info.symtab = 0;
 124        vdso_info.versym = 0;
 125        vdso_info.verdef = 0;
 126        for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
 127                switch (dyn[i].d_tag) {
 128                case DT_STRTAB:
 129                        vdso_info.symstrings = (const char *)
 130                                ((uintptr_t)dyn[i].d_un.d_ptr
 131                                 + vdso_info.load_offset);
 132                        break;
 133                case DT_SYMTAB:
 134                        vdso_info.symtab = (Elf64_Sym *)
 135                                ((uintptr_t)dyn[i].d_un.d_ptr
 136                                 + vdso_info.load_offset);
 137                        break;
 138                case DT_HASH:
 139                        hash = (Elf64_Word *)
 140                                ((uintptr_t)dyn[i].d_un.d_ptr
 141                                 + vdso_info.load_offset);
 142                        break;
 143                case DT_VERSYM:
 144                        vdso_info.versym = (Elf64_Versym *)
 145                                ((uintptr_t)dyn[i].d_un.d_ptr
 146                                 + vdso_info.load_offset);
 147                        break;
 148                case DT_VERDEF:
 149                        vdso_info.verdef = (Elf64_Verdef *)
 150                                ((uintptr_t)dyn[i].d_un.d_ptr
 151                                 + vdso_info.load_offset);
 152                        break;
 153                }
 154        }
 155        if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
 156                return;  /* Failed */
 157
 158        if (!vdso_info.verdef)
 159                vdso_info.versym = 0;
 160
 161        /* Parse the hash table header. */
 162        vdso_info.nbucket = hash[0];
 163        vdso_info.nchain = hash[1];
 164        vdso_info.bucket = &hash[2];
 165        vdso_info.chain = &hash[vdso_info.nbucket + 2];
 166
 167        /* That's all we need. */
 168        vdso_info.valid = true;
 169}
 170
 171static bool vdso_match_version(Elf64_Versym ver,
 172                               const char *name, Elf64_Word hash)
 173{
 174        /*
 175         * This is a helper function to check if the version indexed by
 176         * ver matches name (which hashes to hash).
 177         *
 178         * The version definition table is a mess, and I don't know how
 179         * to do this in better than linear time without allocating memory
 180         * to build an index.  I also don't know why the table has
 181         * variable size entries in the first place.
 182         *
 183         * For added fun, I can't find a comprehensible specification of how
 184         * to parse all the weird flags in the table.
 185         *
 186         * So I just parse the whole table every time.
 187         */
 188
 189        /* First step: find the version definition */
 190        ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
 191        Elf64_Verdef *def = vdso_info.verdef;
 192        while(true) {
 193                if ((def->vd_flags & VER_FLG_BASE) == 0
 194                    && (def->vd_ndx & 0x7fff) == ver)
 195                        break;
 196
 197                if (def->vd_next == 0)
 198                        return false;  /* No definition. */
 199
 200                def = (Elf64_Verdef *)((char *)def + def->vd_next);
 201        }
 202
 203        /* Now figure out whether it matches. */
 204        Elf64_Verdaux *aux = (Elf64_Verdaux*)((char *)def + def->vd_aux);
 205        return def->vd_hash == hash
 206                && !strcmp(name, vdso_info.symstrings + aux->vda_name);
 207}
 208
 209void *vdso_sym(const char *version, const char *name)
 210{
 211        unsigned long ver_hash;
 212        if (!vdso_info.valid)
 213                return 0;
 214
 215        ver_hash = elf_hash(version);
 216        Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
 217
 218        for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
 219                Elf64_Sym *sym = &vdso_info.symtab[chain];
 220
 221                /* Check for a defined global or weak function w/ right name. */
 222                if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
 223                        continue;
 224                if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
 225                    ELF64_ST_BIND(sym->st_info) != STB_WEAK)
 226                        continue;
 227                if (sym->st_shndx == SHN_UNDEF)
 228                        continue;
 229                if (strcmp(name, vdso_info.symstrings + sym->st_name))
 230                        continue;
 231
 232                /* Check symbol version. */
 233                if (vdso_info.versym
 234                    && !vdso_match_version(vdso_info.versym[chain],
 235                                           version, ver_hash))
 236                        continue;
 237
 238                return (void *)(vdso_info.load_offset + sym->st_value);
 239        }
 240
 241        return 0;
 242}
 243
 244void vdso_init_from_auxv(void *auxv)
 245{
 246        Elf64_auxv_t *elf_auxv = auxv;
 247        for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
 248        {
 249                if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
 250                        vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
 251                        return;
 252                }
 253        }
 254
 255        vdso_info.valid = false;
 256}
 257
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