tools/testing/selftests/vDSO/parse_vdso.c

   1 /*
   2  * parse_vdso.c: Linux reference vDSO parser
   3  * Written by Andrew Lutomirski, 2011-2014.
   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.  In principle it should work on any
  15  * architecture that has a vDSO.
  16  */
  17
  18 #include <stdbool.h>
  19 #include <stdint.h>
  20 #include <string.h>
  21 #include <limits.h>
  22 #include <elf.h>
  23
  24 #include "parse_vdso.h"
  25
  26 /* And here's the code. */
  27 #ifndef ELF_BITS
  28 # if ULONG_MAX > 0xffffffffUL
  29 #  define ELF_BITS 64
  30 # else
  31 #  define ELF_BITS 32
  32 # endif
  33 #endif
  34
  35 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
  36 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
  37 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
  38
  39 static struct vdso_info
  40 {
  41         bool valid;
  42
  43         /* Load information */
  44         uintptr_t load_addr;
  45         uintptr_t load_offset;  /* load_addr - recorded vaddr */
  46
  47         /* Symbol table */
  48         ELF(Sym) *symtab;
  49         const char *symstrings;
  50         ELF(Word) *bucket, *chain;
  51         ELF(Word) nbucket, nchain;
  52
  53         /* Version table */
  54         ELF(Versym) *versym;
  55         ELF(Verdef) *verdef;
  56 } vdso_info;
  57
  58 /* Straight from the ELF specification. */
  59 static unsigned long elf_hash(const unsigned char *name)
  60 {
  61         unsigned long h = 0, g;
  62         while (*name)
  63         {
  64                 h = (h << 4) + *name++;
  65                 if (g = h & 0xf0000000)
  66                         h ^= g >> 24;
  67                 h &= ~g;
  68         }
  69         return h;
  70 }
  71
  72 void vdso_init_from_sysinfo_ehdr(uintptr_t base)
  73 {
  74         size_t i;
  75         bool found_vaddr = false;
  76
  77         vdso_info.valid = false;
  78
  79         vdso_info.load_addr = base;
  80
  81         ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
  82         if (hdr->e_ident[EI_CLASS] !=
  83             (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
  84                 return;  /* Wrong ELF class -- check ELF_BITS */
  85         }
  86
  87         ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
  88         ELF(Dyn) *dyn = 0;
  89
  90         /*
  91          * We need two things from the segment table: the load offset
  92          * and the dynamic table.
  93          */
  94         for (i = 0; i < hdr->e_phnum; i++)
  95         {
  96                 if (pt[i].p_type == PT_LOAD && !found_vaddr) {
  97                         found_vaddr = true;
  98                         vdso_info.load_offset = base
  99                                 + (uintptr_t)pt[i].p_offset
 100                                 - (uintptr_t)pt[i].p_vaddr;
 101                 } else if (pt[i].p_type == PT_DYNAMIC) {
 102                         dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
 103                 }
 104         }
 105
 106         if (!found_vaddr || !dyn)
 107                 return;  /* Failed */
 108
 109         /*
 110          * Fish out the useful bits of the dynamic table.
 111          */
 112         ELF(Word) *hash = 0;
 113         vdso_info.symstrings = 0;
 114         vdso_info.symtab = 0;
 115         vdso_info.versym = 0;
 116         vdso_info.verdef = 0;
 117         for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
 118                 switch (dyn[i].d_tag) {
 119                 case DT_STRTAB:
 120                         vdso_info.symstrings = (const char *)
 121                                 ((uintptr_t)dyn[i].d_un.d_ptr
 122                                  + vdso_info.load_offset);
 123                         break;
 124                 case DT_SYMTAB:
 125                         vdso_info.symtab = (ELF(Sym) *)
 126                                 ((uintptr_t)dyn[i].d_un.d_ptr
 127                                  + vdso_info.load_offset);
 128                         break;
 129                 case DT_HASH:
 130                         hash = (ELF(Word) *)
 131                                 ((uintptr_t)dyn[i].d_un.d_ptr
 132                                  + vdso_info.load_offset);
 133                         break;
 134                 case DT_VERSYM:
 135                         vdso_info.versym = (ELF(Versym) *)
 136                                 ((uintptr_t)dyn[i].d_un.d_ptr
 137                                  + vdso_info.load_offset);
 138                         break;
 139                 case DT_VERDEF:
 140                         vdso_info.verdef = (ELF(Verdef) *)
 141                                 ((uintptr_t)dyn[i].d_un.d_ptr
 142                                  + vdso_info.load_offset);
 143                         break;
 144                 }
 145         }
 146         if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
 147                 return;  /* Failed */
 148
 149         if (!vdso_info.verdef)
 150                 vdso_info.versym = 0;
 151
 152         /* Parse the hash table header. */
 153         vdso_info.nbucket = hash[0];
 154         vdso_info.nchain = hash[1];
 155         vdso_info.bucket = &hash[2];
 156         vdso_info.chain = &hash[vdso_info.nbucket + 2];
 157
 158         /* That's all we need. */
 159         vdso_info.valid = true;
 160 }
 161
 162 static bool vdso_match_version(ELF(Versym) ver,
 163                                const char *name, ELF(Word) hash)
 164 {
 165         /*
 166          * This is a helper function to check if the version indexed by
 167          * ver matches name (which hashes to hash).
 168          *
 169          * The version definition table is a mess, and I don't know how
 170          * to do this in better than linear time without allocating memory
 171          * to build an index.  I also don't know why the table has
 172          * variable size entries in the first place.
 173          *
 174          * For added fun, I can't find a comprehensible specification of how
 175          * to parse all the weird flags in the table.
 176          *
 177          * So I just parse the whole table every time.
 178          */
 179
 180         /* First step: find the version definition */
 181         ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
 182         ELF(Verdef) *def = vdso_info.verdef;
 183         while(true) {
 184                 if ((def->vd_flags & VER_FLG_BASE) == 0
 185                     && (def->vd_ndx & 0x7fff) == ver)
 186                         break;
 187
 188                 if (def->vd_next == 0)
 189                         return false;  /* No definition. */
 190
 191                 def = (ELF(Verdef) *)((char *)def + def->vd_next);
 192         }
 193
 194         /* Now figure out whether it matches. */
 195         ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
 196         return def->vd_hash == hash
 197                 && !strcmp(name, vdso_info.symstrings + aux->vda_name);
 198 }
 199
 200 void *vdso_sym(const char *version, const char *name)
 201 {
 202         unsigned long ver_hash;
 203         if (!vdso_info.valid)
 204                 return 0;
 205
 206         ver_hash = elf_hash(version);
 207         ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
 208
 209         for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
 210                 ELF(Sym) *sym = &vdso_info.symtab[chain];
 211
 212                 /* Check for a defined global or weak function w/ right name. */
 213                 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
 214                         continue;
 215                 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
 216                     ELF64_ST_BIND(sym->st_info) != STB_WEAK)
 217                         continue;
 218                 if (sym->st_shndx == SHN_UNDEF)
 219                         continue;
 220                 if (strcmp(name, vdso_info.symstrings + sym->st_name))
 221                         continue;
 222
 223                 /* Check symbol version. */
 224                 if (vdso_info.versym
 225                     && !vdso_match_version(vdso_info.versym[chain],
 226                                            version, ver_hash))
 227                         continue;
 228
 229                 return (void *)(vdso_info.load_offset + sym->st_value);
 230         }
 231
 232         return 0;
 233 }
 234
 235 void vdso_init_from_auxv(void *auxv)
 236 {
 237         ELF(auxv_t) *elf_auxv = auxv;
 238         for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
 239         {
 240                 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
 241                         vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
 242                         return;
 243                 }
 244         }
 245
 246         vdso_info.valid = false;
 247 }