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 #ifdef __s390x__
  40 #define ELF_HASH_ENTRY ELF(Xword)
  41 #else
  42 #define ELF_HASH_ENTRY ELF(Word)
  43 #endif
  44
  45 static struct vdso_info
  46 {
  47         bool valid;
  48
  49         /* Load information */
  50         uintptr_t load_addr;
  51         uintptr_t load_offset;  /* load_addr - recorded vaddr */
  52
  53         /* Symbol table */
  54         ELF(Sym) *symtab;
  55         const char *symstrings;
  56         ELF_HASH_ENTRY *bucket, *chain;
  57         ELF_HASH_ENTRY nbucket, nchain;
  58
  59         /* Version table */
  60         ELF(Versym) *versym;
  61         ELF(Verdef) *verdef;
  62 } vdso_info;
  63
  64 /*
  65  * Straight from the ELF specification...and then tweaked slightly, in order to
  66  * avoid a few clang warnings.
  67  */
  68 static unsigned long elf_hash(const char *name)
  69 {
  70         unsigned long h = 0, g;
  71         const unsigned char *uch_name = (const unsigned char *)name;
  72
  73         while (*uch_name)
  74         {
  75                 h = (h << 4) + *uch_name++;
  76                 g = h & 0xf0000000;
  77                 if (g)
  78                         h ^= g >> 24;
  79                 h &= ~g;
  80         }
  81         return h;
  82 }
  83
  84 void vdso_init_from_sysinfo_ehdr(uintptr_t base)
  85 {
  86         size_t i;
  87         bool found_vaddr = false;
  88
  89         vdso_info.valid = false;
  90
  91         vdso_info.load_addr = base;
  92
  93         ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
  94         if (hdr->e_ident[EI_CLASS] !=
  95             (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
  96                 return;  /* Wrong ELF class -- check ELF_BITS */
  97         }
  98
  99         ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
 100         ELF(Dyn) *dyn = 0;
 101
 102         /*
 103          * We need two things from the segment table: the load offset
 104          * and the dynamic table.
 105          */
 106         for (i = 0; i < hdr->e_phnum; i++)
 107         {
 108                 if (pt[i].p_type == PT_LOAD && !found_vaddr) {
 109                         found_vaddr = true;
 110                         vdso_info.load_offset = base
 111                                 + (uintptr_t)pt[i].p_offset
 112                                 - (uintptr_t)pt[i].p_vaddr;
 113                 } else if (pt[i].p_type == PT_DYNAMIC) {
 114                         dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
 115                 }
 116         }
 117
 118         if (!found_vaddr || !dyn)
 119                 return;  /* Failed */
 120
 121         /*
 122          * Fish out the useful bits of the dynamic table.
 123          */
 124         ELF_HASH_ENTRY *hash = 0;
 125         vdso_info.symstrings = 0;
 126         vdso_info.symtab = 0;
 127         vdso_info.versym = 0;
 128         vdso_info.verdef = 0;
 129         for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
 130                 switch (dyn[i].d_tag) {
 131                 case DT_STRTAB:
 132                         vdso_info.symstrings = (const char *)
 133                                 ((uintptr_t)dyn[i].d_un.d_ptr
 134                                  + vdso_info.load_offset);
 135                         break;
 136                 case DT_SYMTAB:
 137                         vdso_info.symtab = (ELF(Sym) *)
 138                                 ((uintptr_t)dyn[i].d_un.d_ptr
 139                                  + vdso_info.load_offset);
 140                         break;
 141                 case DT_HASH:
 142                         hash = (ELF_HASH_ENTRY *)
 143                                 ((uintptr_t)dyn[i].d_un.d_ptr
 144                                  + vdso_info.load_offset);
 145                         break;
 146                 case DT_VERSYM:
 147                         vdso_info.versym = (ELF(Versym) *)
 148                                 ((uintptr_t)dyn[i].d_un.d_ptr
 149                                  + vdso_info.load_offset);
 150                         break;
 151                 case DT_VERDEF:
 152                         vdso_info.verdef = (ELF(Verdef) *)
 153                                 ((uintptr_t)dyn[i].d_un.d_ptr
 154                                  + vdso_info.load_offset);
 155                         break;
 156                 }
 157         }
 158         if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
 159                 return;  /* Failed */
 160
 161         if (!vdso_info.verdef)
 162                 vdso_info.versym = 0;
 163
 164         /* Parse the hash table header. */
 165         vdso_info.nbucket = hash[0];
 166         vdso_info.nchain = hash[1];
 167         vdso_info.bucket = &hash[2];
 168         vdso_info.chain = &hash[vdso_info.nbucket + 2];
 169
 170         /* That's all we need. */
 171         vdso_info.valid = true;
 172 }
 173
 174 static bool vdso_match_version(ELF(Versym) ver,
 175                                const char *name, ELF(Word) hash)
 176 {
 177         /*
 178          * This is a helper function to check if the version indexed by
 179          * ver matches name (which hashes to hash).
 180          *
 181          * The version definition table is a mess, and I don't know how
 182          * to do this in better than linear time without allocating memory
 183          * to build an index.  I also don't know why the table has
 184          * variable size entries in the first place.
 185          *
 186          * For added fun, I can't find a comprehensible specification of how
 187          * to parse all the weird flags in the table.
 188          *
 189          * So I just parse the whole table every time.
 190          */
 191
 192         /* First step: find the version definition */
 193         ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
 194         ELF(Verdef) *def = vdso_info.verdef;
 195         while(true) {
 196                 if ((def->vd_flags & VER_FLG_BASE) == 0
 197                     && (def->vd_ndx & 0x7fff) == ver)
 198                         break;
 199
 200                 if (def->vd_next == 0)
 201                         return false;  /* No definition. */
 202
 203                 def = (ELF(Verdef) *)((char *)def + def->vd_next);
 204         }
 205
 206         /* Now figure out whether it matches. */
 207         ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
 208         return def->vd_hash == hash
 209                 && !strcmp(name, vdso_info.symstrings + aux->vda_name);
 210 }
 211
 212 void *vdso_sym(const char *version, const char *name)
 213 {
 214         unsigned long ver_hash;
 215         if (!vdso_info.valid)
 216                 return 0;
 217
 218         ver_hash = elf_hash(version);
 219         ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
 220
 221         for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
 222                 ELF(Sym) *sym = &vdso_info.symtab[chain];
 223
 224                 /* Check for a defined global or weak function w/ right name. */
 225                 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
 226                         continue;
 227                 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
 228                     ELF64_ST_BIND(sym->st_info) != STB_WEAK)
 229                         continue;
 230                 if (sym->st_shndx == SHN_UNDEF)
 231                         continue;
 232                 if (strcmp(name, vdso_info.symstrings + sym->st_name))
 233                         continue;
 234
 235                 /* Check symbol version. */
 236                 if (vdso_info.versym
 237                     && !vdso_match_version(vdso_info.versym[chain],
 238                                            version, ver_hash))
 239                         continue;
 240
 241                 return (void *)(vdso_info.load_offset + sym->st_value);
 242         }
 243
 244         return 0;
 245 }
 246
 247 void vdso_init_from_auxv(void *auxv)
 248 {
 249         ELF(auxv_t) *elf_auxv = auxv;
 250         for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
 251         {
 252                 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
 253                         vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
 254                         return;
 255                 }
 256         }
 257
 258         vdso_info.valid = false;
 259 }