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.  FLAGS is a set of flags.  If DL_LOOKUP_NOEXEC is set,
  49    then don't search the executable for a definition; this used for
  50    copy relocs.  If DL_LOOKUP_NOPLT is set, then a PLT entry cannot
  51    satisfy the reference; some different binding must be found.  */
  52
  53 ElfW(Addr)
  54 _dl_lookup_symbol (const char *undef_name, const ElfW(Sym) **ref,
  55                    struct link_map *symbol_scope[],
  56                    const char *reference_name,
  57                    int flags)
  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         /* Don't search the executable when resolving a copy reloc.  */
  79         if (flags & DL_LOOKUP_NOEXEC && map->l_type == lt_executable)
  80           continue;
  81
  82         symtab = ((void *) map->l_addr + map->l_info[DT_SYMTAB]->d_un.d_ptr);
  83         strtab = ((void *) map->l_addr + map->l_info[DT_STRTAB]->d_un.d_ptr);
  84
  85         /* Search the appropriate hash bucket in this object's symbol table
  86            for a definition for the same symbol name.  */
  87         for (symidx = map->l_buckets[hash % map->l_nbuckets];
  88              symidx != STN_UNDEF;
  89              symidx = map->l_chain[symidx])
  90           {
  91             const ElfW(Sym) *sym = &symtab[symidx];
  92
  93             if (sym->st_value == 0 || /* No value.  */
  94                 ((flags & DL_LOOKUP_NOPLT) != 0 /* Reject PLT entry.  */
  95                  && sym->st_shndx == SHN_UNDEF))
  96               continue;
  97
  98             switch (ELFW(ST_TYPE) (sym->st_info))
  99               {
 100               case STT_NOTYPE:
 101               case STT_FUNC:
 102               case STT_OBJECT:
 103                 break;
 104               default:
 105                 /* Not a code/data definition.  */
 106                 continue;
 107               }
 108
 109             if (sym != *ref && strcmp (strtab + sym->st_name, undef_name))
 110               /* Not the symbol we are looking for.  */
 111               continue;
 112
 113             switch (ELFW(ST_BIND) (sym->st_info))
 114               {
 115               case STB_GLOBAL:
 116                 /* Global definition.  Just what we need.  */
 117                 *ref = sym;
 118                 return map->l_addr;
 119               case STB_WEAK:
 120                 /* Weak definition.  Use this value if we don't find
 121                    another.  */
 122                 if (! weak_value.s)
 123                   {
 124                     weak_value.s = sym;
 125                     weak_value.a = map->l_addr;
 126                   }
 127                 break;
 128               default:
 129                 /* Local symbols are ignored.  */
 130                 break;
 131               }
 132           }
 133       }
 134
 135   if (weak_value.s == NULL &&
 136       (*ref == NULL || ELFW(ST_BIND) ((*ref)->st_info) != STB_WEAK))
 137     {
 138       /* We could find no value for a strong reference.  */
 139       const char msg[] = "undefined symbol: ";
 140       const size_t len = strlen (undef_name);
 141       char buf[sizeof msg + len];
 142       memcpy (buf, msg, sizeof msg - 1);
 143       memcpy (&buf[sizeof msg - 1], undef_name, len + 1);
 144       _dl_signal_error (0, reference_name, buf);
 145     }
 146
 147   *ref = weak_value.s;
 148   return weak_value.a;
 149 }
 150
 151
 152 /* Cache the location of MAP's hash table.  */
 153
 154 void
 155 _dl_setup_hash (struct link_map *map)
 156 {
 157   ElfW(Symndx) *hash = (void *)(map->l_addr + map->l_info[DT_HASH]->d_un.d_ptr);
 158   ElfW(Symndx) nchain;
 159   map->l_nbuckets = *hash++;
 160   nchain = *hash++;
 161   map->l_buckets = hash;
 162   hash += map->l_nbuckets;
 163   map->l_chain = hash;
 164 }