Documentation/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 /*
  25  * To use this vDSO parser, first call one of the vdso_init_* functions.
  26  * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
  27  * to vdso_init_from_sysinfo_ehdr.  Otherwise pass auxv to vdso_init_from_auxv.
  28  * Then call vdso_sym for each symbol you want.  For example, to look up
  29  * gettimeofday on x86_64, use:
  30  *
  31  *     <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
  32  * or
  33  *     <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
  34  *
  35  * vdso_sym will return 0 if the symbol doesn't exist or if the init function
  36  * failed or was not called.  vdso_sym is a little slow, so its return value
  37  * should be cached.
  38  *
  39  * vdso_sym is threadsafe; the init functions are not.
  40  *
  41  * These are the prototypes:
  42  */
  43 extern void vdso_init_from_auxv(void *auxv);
  44 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
  45 extern void *vdso_sym(const char *version, const char *name);
  46
  47
  48 /* And here's the code. */
  49 #ifndef ELF_BITS
  50 # if ULONG_MAX > 0xffffffffUL
  51 #  define ELF_BITS 64
  52 # else
  53 #  define ELF_BITS 32
  54 # endif
  55 #endif
  56
  57 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
  58 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
  59 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
  60
  61 static struct vdso_info
  62 {
  63         bool valid;
  64
  65         /* Load information */
  66         uintptr_t load_addr;
  67         uintptr_t load_offset;  /* load_addr - recorded vaddr */
  68
  69         /* Symbol table */
  70         ELF(Sym) *symtab;
  71         const char *symstrings;
  72         ELF(Word) *bucket, *chain;
  73         ELF(Word) nbucket, nchain;
  74
  75         /* Version table */
  76         ELF(Versym) *versym;
  77         ELF(Verdef) *verdef;
  78 } vdso_info;
  79
  80 /* Straight from the ELF specification. */
  81 static unsigned long elf_hash(const unsigned char *name)
  82 {
  83         unsigned long h = 0, g;
  84         while (*name)
  85         {
  86                 h = (h << 4) + *name++;
  87                 if (g = h & 0xf0000000)
  88                         h ^= g >> 24;
  89                 h &= ~g;
  90         }
  91         return h;
  92 }
  93
  94 void vdso_init_from_sysinfo_ehdr(uintptr_t base)
  95 {
  96         size_t i;
  97         bool found_vaddr = false;
  98
  99         vdso_info.valid = false;
 100
 101         vdso_info.load_addr = base;
 102
 103         ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
 104         if (hdr->e_ident[EI_CLASS] !=
 105             (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
 106                 return;  /* Wrong ELF class -- check ELF_BITS */
 107         }
 108
 109         ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
 110         ELF(Dyn) *dyn = 0;
 111
 112         /*
 113          * We need two things from the segment table: the load offset
 114          * and the dynamic table.
 115          */
 116         for (i = 0; i < hdr->e_phnum; i++)
 117         {
 118                 if (pt[i].p_type == PT_LOAD && !found_vaddr) {
 119                         found_vaddr = true;
 120                         vdso_info.load_offset = base
 121                                 + (uintptr_t)pt[i].p_offset
 122                                 - (uintptr_t)pt[i].p_vaddr;
 123                 } else if (pt[i].p_type == PT_DYNAMIC) {
 124                         dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
 125                 }
 126         }
 127
 128         if (!found_vaddr || !dyn)
 129                 return;  /* Failed */
 130
 131         /*
 132          * Fish out the useful bits of the dynamic table.
 133          */
 134         ELF(Word) *hash = 0;
 135         vdso_info.symstrings = 0;
 136         vdso_info.symtab = 0;
 137         vdso_info.versym = 0;
 138         vdso_info.verdef = 0;
 139         for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
 140                 switch (dyn[i].d_tag) {
 141                 case DT_STRTAB:
 142                         vdso_info.symstrings = (const char *)
 143                                 ((uintptr_t)dyn[i].d_un.d_ptr
 144                                  + vdso_info.load_offset);
 145                         break;
 146                 case DT_SYMTAB:
 147                         vdso_info.symtab = (ELF(Sym) *)
 148                                 ((uintptr_t)dyn[i].d_un.d_ptr
 149                                  + vdso_info.load_offset);
 150                         break;
 151                 case DT_HASH:
 152                         hash = (ELF(Word) *)
 153                                 ((uintptr_t)dyn[i].d_un.d_ptr
 154                                  + vdso_info.load_offset);
 155                         break;
 156                 case DT_VERSYM:
 157                         vdso_info.versym = (ELF(Versym) *)
 158                                 ((uintptr_t)dyn[i].d_un.d_ptr
 159                                  + vdso_info.load_offset);
 160                         break;
 161                 case DT_VERDEF:
 162                         vdso_info.verdef = (ELF(Verdef) *)
 163                                 ((uintptr_t)dyn[i].d_un.d_ptr
 164                                  + vdso_info.load_offset);
 165                         break;
 166                 }
 167         }
 168         if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
 169                 return;  /* Failed */
 170
 171         if (!vdso_info.verdef)
 172                 vdso_info.versym = 0;
 173
 174         /* Parse the hash table header. */
 175         vdso_info.nbucket = hash[0];
 176         vdso_info.nchain = hash[1];
 177         vdso_info.bucket = &hash[2];
 178         vdso_info.chain = &hash[vdso_info.nbucket + 2];
 179
 180         /* That's all we need. */
 181         vdso_info.valid = true;
 182 }
 183
 184 static bool vdso_match_version(ELF(Versym) ver,
 185                                const char *name, ELF(Word) hash)
 186 {
 187         /*
 188          * This is a helper function to check if the version indexed by
 189          * ver matches name (which hashes to hash).
 190          *
 191          * The version definition table is a mess, and I don't know how
 192          * to do this in better than linear time without allocating memory
 193          * to build an index.  I also don't know why the table has
 194          * variable size entries in the first place.
 195          *
 196          * For added fun, I can't find a comprehensible specification of how
 197          * to parse all the weird flags in the table.
 198          *
 199          * So I just parse the whole table every time.
 200          */
 201
 202         /* First step: find the version definition */
 203         ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
 204         ELF(Verdef) *def = vdso_info.verdef;
 205         while(true) {
 206                 if ((def->vd_flags & VER_FLG_BASE) == 0
 207                     && (def->vd_ndx & 0x7fff) == ver)
 208                         break;
 209
 210                 if (def->vd_next == 0)
 211                         return false;  /* No definition. */
 212
 213                 def = (ELF(Verdef) *)((char *)def + def->vd_next);
 214         }
 215
 216         /* Now figure out whether it matches. */
 217         ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
 218         return def->vd_hash == hash
 219                 && !strcmp(name, vdso_info.symstrings + aux->vda_name);
 220 }
 221
 222 void *vdso_sym(const char *version, const char *name)
 223 {
 224         unsigned long ver_hash;
 225         if (!vdso_info.valid)
 226                 return 0;
 227
 228         ver_hash = elf_hash(version);
 229         ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
 230
 231         for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
 232                 ELF(Sym) *sym = &vdso_info.symtab[chain];
 233
 234                 /* Check for a defined global or weak function w/ right name. */
 235                 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
 236                         continue;
 237                 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
 238                     ELF64_ST_BIND(sym->st_info) != STB_WEAK)
 239                         continue;
 240                 if (sym->st_shndx == SHN_UNDEF)
 241                         continue;
 242                 if (strcmp(name, vdso_info.symstrings + sym->st_name))
 243                         continue;
 244
 245                 /* Check symbol version. */
 246                 if (vdso_info.versym
 247                     && !vdso_match_version(vdso_info.versym[chain],
 248                                            version, ver_hash))
 249                         continue;
 250
 251                 return (void *)(vdso_info.load_offset + sym->st_value);
 252         }
 253
 254         return 0;
 255 }
 256
 257 void vdso_init_from_auxv(void *auxv)
 258 {
 259         ELF(auxv_t) *elf_auxv = auxv;
 260         for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
 261         {
 262                 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
 263                         vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
 264                         return;
 265                 }
 266         }
 267
 268         vdso_info.valid = false;
 269 }