elf/do-lookup.h

   1 /* Look up a symbol in the loaded objects.
   2    Copyright (C) 1995-2007, 2008, 2009 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, write to the Free
  17    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  18    02111-1307 USA.  */
  19
  20
  21 /* Inner part of the lookup functions.  We return a value > 0 if we
  22    found the symbol, the value 0 if nothing is found and < 0 if
  23    something bad happened.  */
  24 static int
  25 __attribute_noinline__
  26 do_lookup_x (const char *undef_name, uint_fast32_t new_hash,
  27              unsigned long int *old_hash, const ElfW(Sym) *ref,
  28              struct sym_val *result, struct r_scope_elem *scope, size_t i,
  29              const struct r_found_version *const version, int flags,
  30              struct link_map *skip, int type_class)
  31 {
  32   size_t n = scope->r_nlist;
  33   /* Make sure we read the value before proceeding.  Otherwise we
  34      might use r_list pointing to the initial scope and r_nlist being
  35      the value after a resize.  That is the only path in dl-open.c not
  36      protected by GSCOPE.  A read barrier here might be to expensive.  */
  37   __asm volatile ("" : "+r" (n), "+m" (scope->r_list));
  38   struct link_map **list = scope->r_list;
  39
  40   do
  41     {
  42       /* These variables are used in the nested function.  */
  43       Elf_Symndx symidx;
  44       int num_versions = 0;
  45       const ElfW(Sym) *versioned_sym = NULL;
  46
  47       const struct link_map *map = list[i]->l_real;
  48
  49       /* Here come the extra test needed for `_dl_lookup_symbol_skip'.  */
  50       if (map == skip)
  51         continue;
  52
  53       /* Don't search the executable when resolving a copy reloc.  */
  54       if ((type_class & ELF_RTYPE_CLASS_COPY) && map->l_type == lt_executable)
  55         continue;
  56
  57       /* Do not look into objects which are going to be removed.  */
  58       if (map->l_removed)
  59         continue;
  60
  61       /* Print some debugging info if wanted.  */
  62       if (__builtin_expect (GLRO(dl_debug_mask) & DL_DEBUG_SYMBOLS, 0))
  63         _dl_debug_printf ("symbol=%s;  lookup in file=%s [%lu]\n",
  64                           undef_name,
  65                           map->l_name[0] ? map->l_name : rtld_progname,
  66                           map->l_ns);
  67
  68       /* If the hash table is empty there is nothing to do here.  */
  69       if (map->l_nbuckets == 0)
  70         continue;
  71
  72       /* The tables for this map.  */
  73       const ElfW(Sym) *symtab = (const void *) D_PTR (map, l_info[DT_SYMTAB]);
  74       const char *strtab = (const void *) D_PTR (map, l_info[DT_STRTAB]);
  75
  76
  77       /* Nested routine to check whether the symbol matches.  */
  78       const ElfW(Sym) *
  79       __attribute_noinline__
  80       check_match (const ElfW(Sym) *sym)
  81       {
  82         unsigned int stt = ELFW(ST_TYPE) (sym->st_info);
  83         assert (ELF_RTYPE_CLASS_PLT == 1);
  84         if (__builtin_expect ((sym->st_value == 0 /* No value.  */
  85                                && stt != STT_TLS)
  86                               || (type_class & (sym->st_shndx == SHN_UNDEF)),
  87                               0))
  88           return NULL;
  89
  90         if (__builtin_expect (stt > STT_FUNC
  91                               && stt != STT_COMMON
  92                               && stt != STT_TLS
  93                               && stt != STT_GNU_IFUNC, 0))
  94           /* Ignore all but STT_NOTYPE, STT_OBJECT, STT_FUNC, STT_COMMON,
  95              STT_TLS, and STT_GNU_IFUNC since these are no code/data
  96              definitions.  */
  97           return NULL;
  98
  99         if (sym != ref && strcmp (strtab + sym->st_name, undef_name))
 100           /* Not the symbol we are looking for.  */
 101           return NULL;
 102
 103         const ElfW(Half) *verstab = map->l_versyms;
 104         if (version != NULL)
 105           {
 106             if (__builtin_expect (verstab == NULL, 0))
 107               {
 108                 /* We need a versioned symbol but haven't found any.  If
 109                    this is the object which is referenced in the verneed
 110                    entry it is a bug in the library since a symbol must
 111                    not simply disappear.
 112
 113                    It would also be a bug in the object since it means that
 114                    the list of required versions is incomplete and so the
 115                    tests in dl-version.c haven't found a problem.*/
 116                 assert (version->filename == NULL
 117                         || ! _dl_name_match_p (version->filename, map));
 118
 119                 /* Otherwise we accept the symbol.  */
 120               }
 121             else
 122               {
 123                 /* We can match the version information or use the
 124                    default one if it is not hidden.  */
 125                 ElfW(Half) ndx = verstab[symidx] & 0x7fff;
 126                 if ((map->l_versions[ndx].hash != version->hash
 127                      || strcmp (map->l_versions[ndx].name, version->name))
 128                     && (version->hidden || map->l_versions[ndx].hash
 129                         || (verstab[symidx] & 0x8000)))
 130                   /* It's not the version we want.  */
 131                   return NULL;
 132               }
 133           }
 134         else
 135           {
 136             /* No specific version is selected.  There are two ways we
 137                can got here:
 138
 139                - a binary which does not include versioning information
 140                is loaded
 141
 142                - dlsym() instead of dlvsym() is used to get a symbol which
 143                might exist in more than one form
 144
 145                If the library does not provide symbol version information
 146                there is no problem at at: we simply use the symbol if it
 147                is defined.
 148
 149                These two lookups need to be handled differently if the
 150                library defines versions.  In the case of the old
 151                unversioned application the oldest (default) version
 152                should be used.  In case of a dlsym() call the latest and
 153                public interface should be returned.  */
 154             if (verstab != NULL)
 155               {
 156                 if ((verstab[symidx] & 0x7fff)
 157                     >= ((flags & DL_LOOKUP_RETURN_NEWEST) ? 2 : 3))
 158                   {
 159                     /* Don't accept hidden symbols.  */
 160                     if ((verstab[symidx] & 0x8000) == 0
 161                         && num_versions++ == 0)
 162                       /* No version so far.  */
 163                       versioned_sym = sym;
 164
 165                     return NULL;
 166                   }
 167               }
 168           }
 169
 170         /* There cannot be another entry for this symbol so stop here.  */
 171         return sym;
 172       }
 173
 174       const ElfW(Sym) *sym;
 175       const ElfW(Addr) *bitmask = map->l_gnu_bitmask;
 176       if (__builtin_expect (bitmask != NULL, 1))
 177         {
 178           ElfW(Addr) bitmask_word
 179             = bitmask[(new_hash / __ELF_NATIVE_CLASS)
 180                       & map->l_gnu_bitmask_idxbits];
 181
 182           unsigned int hashbit1 = new_hash & (__ELF_NATIVE_CLASS - 1);
 183           unsigned int hashbit2 = ((new_hash >> map->l_gnu_shift)
 184                                    & (__ELF_NATIVE_CLASS - 1));
 185
 186           if (__builtin_expect ((bitmask_word >> hashbit1)
 187                                 & (bitmask_word >> hashbit2) & 1, 0))
 188             {
 189               Elf32_Word bucket = map->l_gnu_buckets[new_hash
 190                                                      % map->l_nbuckets];
 191               if (bucket != 0)
 192                 {
 193                   const Elf32_Word *hasharr = &map->l_gnu_chain_zero[bucket];
 194
 195                   do
 196                     if (((*hasharr ^ new_hash) >> 1) == 0)
 197                       {
 198                         symidx = hasharr - map->l_gnu_chain_zero;
 199                         sym = check_match (&symtab[symidx]);
 200                         if (sym != NULL)
 201                           goto found_it;
 202                       }
 203                   while ((*hasharr++ & 1u) == 0);
 204                 }
 205             }
 206           /* No symbol found.  */
 207           symidx = SHN_UNDEF;
 208         }
 209       else
 210         {
 211           if (*old_hash == 0xffffffff)
 212             *old_hash = _dl_elf_hash (undef_name);
 213
 214           /* Use the old SysV-style hash table.  Search the appropriate
 215              hash bucket in this object's symbol table for a definition
 216              for the same symbol name.  */
 217           for (symidx = map->l_buckets[*old_hash % map->l_nbuckets];
 218                symidx != STN_UNDEF;
 219                symidx = map->l_chain[symidx])
 220             {
 221               sym = check_match (&symtab[symidx]);
 222               if (sym != NULL)
 223                 goto found_it;
 224             }
 225         }
 226
 227       /* If we have seen exactly one versioned symbol while we are
 228          looking for an unversioned symbol and the version is not the
 229          default version we still accept this symbol since there are
 230          no possible ambiguities.  */
 231       sym = num_versions == 1 ? versioned_sym : NULL;
 232
 233       if (sym != NULL)
 234         {
 235         found_it:
 236           switch (ELFW(ST_BIND) (sym->st_info))
 237             {
 238             case STB_WEAK:
 239               /* Weak definition.  Use this value if we don't find another.  */
 240               if (__builtin_expect (GLRO(dl_dynamic_weak), 0))
 241                 {
 242                   if (! result->s)
 243                     {
 244                       result->s = sym;
 245                       result->m = (struct link_map *) map;
 246                     }
 247                   break;
 248                 }
 249               /* FALLTHROUGH */
 250             case STB_GLOBAL:
 251               /* Global definition.  Just what we need.  */
 252               result->s = sym;
 253               result->m = (struct link_map *) map;
 254               return 1;
 255             default:
 256               /* Local symbols are ignored.  */
 257               break;
 258             }
 259         }
 260
 261       /* If this current map is the one mentioned in the verneed entry
 262          and we have not found a weak entry, it is a bug.  */
 263       if (symidx == STN_UNDEF && version != NULL && version->filename != NULL
 264           && __builtin_expect (_dl_name_match_p (version->filename, map), 0))
 265         return -1;
 266     }
 267   while (++i < n);
 268
 269   /* We have not found anything until now.  */
 270   return 0;
 271 }