elf/dl-lookup.c

   1 /* Look up a symbol in the loaded objects.
   2    Copyright (C) 1995, 1996 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 Library General Public License as
   7    published by the Free Software Foundation; either version 2 of the
   8    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    Library General Public License for more details.
  14
  15    You should have received a copy of the GNU Library General Public
  16    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  17    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  18    Boston, MA 02111-1307, USA.  */
  19
  20 #include <stddef.h>
  21 #include <link.h>
  22 #include <assert.h>
  23 #include <string.h>
  24
  25
  26 struct sym_val
  27   {
  28     ElfW(Addr) a;
  29     const ElfW(Sym) *s;
  30   };
  31
  32
  33 /* This is the hashing function specified by the ELF ABI.  */
  34 static inline unsigned
  35 _dl_elf_hash (const char *name)
  36 {
  37   unsigned long int hash = 0;
  38   while (*name != '\0')
  39     {
  40       unsigned long int hi;
  41       hash = (hash << 4) + *name++;
  42       hi = hash & 0xf0000000;
  43       if (hi != 0)
  44         {
  45           hash ^= hi >> 24;
  46           /* The ELF ABI says `hash &= ~hi', but this is equivalent
  47              in this case and on some machines one insn instead of two.  */
  48           hash ^= hi;
  49         }
  50     }
  51   return hash;
  52 }
  53
  54
  55 /* Inner part of the lookup functions.  */
  56 static inline ElfW(Addr)
  57 do_lookup (const char *undef_name, unsigned long int hash,
  58            const ElfW(Sym) **ref, struct sym_val *result,
  59            struct link_map *list[], size_t i, size_t n,
  60            const char *reference_name, struct link_map *skip, int flags)
  61 {
  62   struct link_map *map;
  63
  64   for (; i < n; ++i)
  65     {
  66       const ElfW(Sym) *symtab;
  67       const char *strtab;
  68       ElfW(Symndx) symidx;
  69
  70       map = list[i];
  71
  72       /* Here come the extra test needed for `_dl_lookup_symbol_skip'.  */
  73       if (skip != NULL && map == skip)
  74         continue;
  75
  76       /* Don't search the executable when resolving a copy reloc.  */
  77       if (flags & DL_LOOKUP_NOEXEC && map->l_type == lt_executable)
  78         continue;
  79
  80       symtab = ((void *) map->l_addr + map->l_info[DT_SYMTAB]->d_un.d_ptr);
  81       strtab = ((void *) map->l_addr + map->l_info[DT_STRTAB]->d_un.d_ptr);
  82
  83       /* Search the appropriate hash bucket in this object's symbol table
  84          for a definition for the same symbol name.  */
  85       for (symidx = map->l_buckets[hash % map->l_nbuckets];
  86            symidx != STN_UNDEF;
  87            symidx = map->l_chain[symidx])
  88         {
  89           const ElfW(Sym) *sym = &symtab[symidx];
  90
  91           if (sym->st_value == 0 || /* No value.  */
  92               ((flags & DL_LOOKUP_NOPLT) != 0 /* Reject PLT entry.  */
  93                && sym->st_shndx == SHN_UNDEF))
  94             continue;
  95
  96           switch (ELFW(ST_TYPE) (sym->st_info))
  97             {
  98             case STT_NOTYPE:
  99             case STT_FUNC:
 100             case STT_OBJECT:
 101               break;
 102             default:
 103               /* Not a code/data definition.  */
 104               continue;
 105             }
 106
 107           if (sym != *ref && strcmp (strtab + sym->st_name, undef_name))
 108             /* Not the symbol we are looking for.  */
 109             continue;
 110
 111           switch (ELFW(ST_BIND) (sym->st_info))
 112             {
 113             case STB_GLOBAL:
 114               /* Global definition.  Just what we need.  */
 115               result->s = sym;
 116               result->a = map->l_addr;
 117               return 1;
 118             case STB_WEAK:
 119               /* Weak definition.  Use this value if we don't find
 120                  another.  */
 121               if (! result->s)
 122                 {
 123                   result->s = sym;
 124                   result->a = map->l_addr;
 125                 }
 126               break;
 127             default:
 128               /* Local symbols are ignored.  */
 129               break;
 130             }
 131         }
 132     }
 133
 134   /* We have not found anything until now.  */
 135   return 0;
 136 }
 137
 138 /* Search loaded objects' symbol tables for a definition of the symbol
 139    UNDEF_NAME.  FLAGS is a set of flags.  If DL_LOOKUP_NOEXEC is set,
 140    then don't search the executable for a definition; this used for
 141    copy relocs.  If DL_LOOKUP_NOPLT is set, then a PLT entry cannot
 142    satisfy the reference; some different binding must be found.  */
 143
 144 ElfW(Addr)
 145 _dl_lookup_symbol (const char *undef_name, const ElfW(Sym) **ref,
 146                    struct link_map *symbol_scope[],
 147                    const char *reference_name,
 148                    int flags)
 149 {
 150   const unsigned long int hash = _dl_elf_hash (undef_name);
 151   struct sym_val current_value = { 0, NULL };
 152   struct link_map **scope;
 153
 154   /* Search the relevant loaded objects for a definition.  */
 155   for (scope = symbol_scope; *scope; ++scope)
 156     if (do_lookup (undef_name, hash, ref, &current_value,
 157                    (*scope)->l_searchlist, 0, (*scope)->l_nsearchlist,
 158                    reference_name, NULL, flags))
 159       break;
 160
 161   if (current_value.s == NULL &&
 162       (*ref == NULL || ELFW(ST_BIND) ((*ref)->st_info) != STB_WEAK))
 163     {
 164       /* We could find no value for a strong reference.  */
 165       const char msg[] = "undefined symbol: ";
 166       const size_t len = strlen (undef_name);
 167       char buf[sizeof msg + len];
 168       memcpy (buf, msg, sizeof msg - 1);
 169       memcpy (&buf[sizeof msg - 1], undef_name, len + 1);
 170       _dl_signal_error (0, reference_name, buf);
 171     }
 172
 173   *ref = current_value.s;
 174   return current_value.a;
 175 }
 176
 177
 178 /* This function is nearly the same as `_dl_lookup_symbol' but it
 179    skips in the first list all objects until SKIP_MAP is found.  I.e.,
 180    it only considers objects which were loaded after the described
 181    object.  If there are more search lists the object described by
 182    SKIP_MAP is only skipped.  */
 183 ElfW(Addr)
 184 _dl_lookup_symbol_skip (const char *undef_name, const ElfW(Sym) **ref,
 185                         struct link_map *symbol_scope[],
 186                         const char *reference_name,
 187                         struct link_map *skip_map,
 188                         int flags)
 189 {
 190   const unsigned long int hash = _dl_elf_hash (undef_name);
 191   struct sym_val current_value = { 0, NULL };
 192   struct link_map **scope;
 193   size_t i;
 194
 195   /* Search the relevant loaded objects for a definition.  */
 196   scope = symbol_scope;
 197   for (i = 0; (*scope)->l_dupsearchlist[i] != skip_map; ++i)
 198     assert (i < (*scope)->l_ndupsearchlist);
 199
 200   if (! do_lookup (undef_name, hash, ref, &current_value,
 201                    (*scope)->l_dupsearchlist, i, (*scope)->l_ndupsearchlist,
 202                    reference_name, skip_map, flags))
 203     while (*++scope)
 204       if (do_lookup (undef_name, hash, ref, &current_value,
 205                      (*scope)->l_dupsearchlist, 0, (*scope)->l_ndupsearchlist,
 206                      reference_name, skip_map, flags))
 207         break;
 208
 209   *ref = current_value.s;
 210   return current_value.a;
 211 }
 212
 213
 214 /* Cache the location of MAP's hash table.  */
 215
 216 void
 217 _dl_setup_hash (struct link_map *map)
 218 {
 219   ElfW(Symndx) *hash = (void *)(map->l_addr + map->l_info[DT_HASH]->d_un.d_ptr);
 220   ElfW(Symndx) nchain;
 221   map->l_nbuckets = *hash++;
 222   nchain = *hash++;
 223   map->l_buckets = hash;
 224   hash += map->l_nbuckets;
 225   map->l_chain = hash;
 226 }