Documentation/vDSO/parse_vdso.c

   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  */
  42 extern void vdso_init_from_auxv(void *auxv);
  43 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
  44 extern 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
  53 static 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. */
  73 static 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
  86 void 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
 171 static 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
 209 void *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
 244 void 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 }