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
  17 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  18 Cambridge, MA 02139, USA.  */
  19
  20 #include <stddef.h>
  21 #include <link.h>
  22 #include <assert.h>
  23 #include <string.h>
  24
  25
  26 /* This is the hashing function specified by the ELF ABI.  */
  27 static inline unsigned
  28 _dl_elf_hash (const char *name)
  29 {
  30   unsigned long int hash = 0;
  31   while (*name != '\0')
  32     {
  33       unsigned long int hi;
  34       hash = (hash << 4) + *name++;
  35       hi = hash & 0xf0000000;
  36       if (hi != 0)
  37         {
  38           hash ^= hi >> 24;
  39           /* The ELF ABI says `hash &= ~hi', but this is equivalent
  40              in this case and on some machines one insn instead of two.  */
  41           hash ^= hi;
  42         }
  43     }
  44   return hash;
  45 }
  46
  47 /* Search loaded objects' symbol tables for a definition of the symbol
  48    UNDEF_NAME.  The chosen value can't be RELOC_ADDR.  If NOPLT is nonzero,
  49    then a PLT entry cannot satisfy the reference; some different binding
  50    must be found.  */
  51
  52 ElfW(Addr)
  53 _dl_lookup_symbol (const char *undef_name, const ElfW(Sym) **ref,
  54                    struct link_map *symbol_scope[],
  55                    const char *reference_name,
  56                    ElfW(Addr) reloc_addr,
  57                    int noplt)
  58 {
  59   const unsigned long int hash = _dl_elf_hash (undef_name);
  60   struct
  61     {
  62       ElfW(Addr) a;
  63       const ElfW(Sym) *s;
  64     } weak_value = { 0, NULL };
  65   size_t i;
  66   struct link_map **scope, *map;
  67
  68   /* Search the relevant loaded objects for a definition.  */
  69   for (scope = symbol_scope; *scope; ++scope)
  70     for (i = 0; i < (*scope)->l_nsearchlist; ++i)
  71       {
  72         const ElfW(Sym) *symtab;
  73         const char *strtab;
  74         ElfW(Symndx) symidx;
  75
  76         map = (*scope)->l_searchlist[i];
  77
  78         symtab = ((void *) map->l_addr + map->l_info[DT_SYMTAB]->d_un.d_ptr);
  79         strtab = ((void *) map->l_addr + map->l_info[DT_STRTAB]->d_un.d_ptr);
  80
  81         /* Search the appropriate hash bucket in this object's symbol table
  82            for a definition for the same symbol name.  */
  83         for (symidx = map->l_buckets[hash % map->l_nbuckets];
  84              symidx != STN_UNDEF;
  85              symidx = map->l_chain[symidx])
  86           {
  87             const ElfW(Sym) *sym = &symtab[symidx];
  88
  89             if (sym->st_value == 0 || /* No value.  */
  90                 /* Cannot resolve to the location being filled in.  */
  91                 reloc_addr == map->l_addr + sym->st_value ||
  92                 (noplt && sym->st_shndx == SHN_UNDEF)) /* Reject PLT.  */
  93               continue;
  94
  95             switch (ELFW(ST_TYPE) (sym->st_info))
  96               {
  97               case STT_NOTYPE:
  98               case STT_FUNC:
  99               case STT_OBJECT:
 100                 break;
 101               default:
 102                 /* Not a code/data definition.  */
 103                 continue;
 104               }
 105
 106             if (sym != *ref && strcmp (strtab + sym->st_name, undef_name))
 107               /* Not the symbol we are looking for.  */
 108               continue;
 109
 110             switch (ELFW(ST_BIND) (sym->st_info))
 111               {
 112               case STB_GLOBAL:
 113                 /* Global definition.  Just what we need.  */
 114                 *ref = sym;
 115                 return map->l_addr;
 116               case STB_WEAK:
 117                 /* Weak definition.  Use this value if we don't find
 118                    another.  */
 119                 if (! weak_value.s)
 120                   {
 121                     weak_value.s = sym;
 122                     weak_value.a = map->l_addr;
 123                   }
 124                 break;
 125               default:
 126                 /* Local symbols are ignored.  */
 127                 break;
 128               }
 129           }
 130       }
 131
 132   if (weak_value.s == NULL &&
 133       (*ref == NULL || ELFW(ST_BIND) ((*ref)->st_info) != STB_WEAK))
 134     {
 135       /* We could find no value for a strong reference.  */
 136       const char msg[] = "undefined symbol: ";
 137       const size_t len = strlen (undef_name);
 138       char buf[sizeof msg + len];
 139       memcpy (buf, msg, sizeof msg - 1);
 140       memcpy (&buf[sizeof msg - 1], undef_name, len + 1);
 141       _dl_signal_error (0, reference_name, buf);
 142     }
 143
 144   *ref = weak_value.s;
 145   return weak_value.a;
 146 }
 147
 148
 149 /* Cache the location of MAP's hash table.  */
 150
 151 void
 152 _dl_setup_hash (struct link_map *map)
 153 {
 154   ElfW(Symndx) *hash = (void *)(map->l_addr + map->l_info[DT_HASH]->d_un.d_ptr);
 155   ElfW(Symndx) nchain;
 156   map->l_nbuckets = *hash++;
 157   nchain = *hash++;
 158   map->l_buckets = hash;
 159   hash += map->l_nbuckets;
 160   map->l_chain = hash;
 161 }