glibc/elf/dl-lookup.c

/* Look up a symbol in the loaded objects.
Copyright (C) 1995, 1996 Free Software Foundation, Inc.
This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.  */

#include <stddef.h>
#include <libelf.h>
#include <link.h>
#include <assert.h>

/* Search loaded objects' symbol tables for a definition of the symbol
   UNDEF_NAME.  The chosen value can't be RELOC_ADDR.  If NOPLT is nonzero,
   then a PLT entry cannot satisfy the reference; some different binding
   must be found.  */

Elf32_Addr
_dl_lookup_symbol (const char *undef_name, const Elf32_Sym **ref,
		   struct link_map *symbol_scope[2],
		   const char *reference_name,
		   Elf32_Addr reloc_addr,
		   int noplt)
{
  unsigned long int hash = elf_hash (undef_name);
  struct
    {
      Elf32_Addr a;
      const Elf32_Sym *s;
    } weak_value = { 0, NULL };
  size_t i;
  struct link_map **scope, *map;

  /* Search the relevant loaded objects for a definition.  */
  for (scope = symbol_scope; scope < &symbol_scope[2]; ++scope)
    if (*scope)
      for (i = 0; i < (*scope)->l_nsearchlist; ++i)
	{
	  const Elf32_Sym *symtab;
	  const char *strtab;
	  Elf32_Word symidx;

	  map = (*scope)->l_searchlist[i];

	  symtab = ((void *) map->l_addr + map->l_info[DT_SYMTAB]->d_un.d_ptr);
	  strtab = ((void *) map->l_addr + map->l_info[DT_STRTAB]->d_un.d_ptr);

	  /* Search the appropriate hash bucket in this object's symbol table
	     for a definition for the same symbol name.  */
	  for (symidx = map->l_buckets[hash % map->l_nbuckets];
	       symidx != STN_UNDEF;
	       symidx = map->l_chain[symidx])
	    {
	      const Elf32_Sym *sym = &symtab[symidx];

	      if (sym->st_value == 0 || /* No value.  */
		  /* Cannot resolve to the location being filled in.  */
		  reloc_addr == map->l_addr + sym->st_value ||
		  (noplt && sym->st_shndx == SHN_UNDEF)) /* Reject PLT.  */
		continue;

	      switch (ELF32_ST_TYPE (sym->st_info))
		{
		case STT_NOTYPE:
		case STT_FUNC:
		case STT_OBJECT:
		  break;
		default:
		  /* Not a code/data definition.  */
		  continue;
		}

	      if (sym != *ref && strcmp (strtab + sym->st_name, undef_name))
		/* Not the symbol we are looking for.  */
		continue;

	      switch (ELF32_ST_BIND (sym->st_info))
		{
		case STB_GLOBAL:
		  /* Global definition.  Just what we need.  */
		  *ref = sym;
		  return map->l_addr;
		case STB_WEAK:
		  /* Weak definition.  Use this value if we don't find
		     another.  */
		  if (! weak_value.s)
		    {
		      weak_value.s = sym;
		      weak_value.a = map->l_addr;
		    }
		  break;
		default:
		  /* Local symbols are ignored.  */
		  break;
		}
	    }
	}

  if (weak_value.s == NULL && ELF32_ST_BIND ((*ref)->st_info) != STB_WEAK)
    {
      /* The symbol was not defined by any object in scope.  To allow
	 access to dynamic linker functionality without using -ldl and
	 thereby brining the dynamic linker's symbols into scope, we
	 recognize a few magical symbol names and resolve them to the
	 addresses of functions inside the dynamic linker.  */

      struct magic
	{
	  unsigned long int hash;
	  const char *name;
	  Elf32_Addr value;
	};
      static struct magic magic[] =
	{
	  { 0xd6a2a5e, "_GNU_libc_dl_open", (Elf32_Addr) &_dl_open },
	  { 0x69ef845, "_GNU_libc_dl_close", (Elf32_Addr) &_dl_close },
	  { 0xae4d63c, "_GNU_libc_dl_symbol", (Elf32_Addr) &_dl_symbol_value },
	  { 0, NULL, 0 }
	};
      struct magic *m;

      for (m = magic; m->hash; ++m)
	if (hash == m->hash && !strcmp (name, m->name))
	  return m->value;

      {
	const char msg[] = "undefined symbol: ";
	char buf[sizeof msg + strlen (undef_name)];
	memcpy (buf, msg, sizeof msg - 1);
	memcpy (&buf[sizeof msg - 1], undef_name,
		sizeof buf - sizeof msg + 1);
	_dl_signal_error (0, reference_name, buf);
      }
    }

  *ref = weak_value.s;
  return weak_value.a;
}


/* Cache the location of MAP's hash table.  */

void
_dl_setup_hash (struct link_map *map)
{
  Elf32_Word *hash = (void *) map->l_addr + map->l_info[DT_HASH]->d_un.d_ptr;
  Elf32_Word nchain;
  map->l_nbuckets = *hash++;
  nchain = *hash++;
  map->l_buckets = hash;
  hash += map->l_nbuckets;
  map->l_chain = hash;
}

/* Look up symbol NAME in MAP's scope and return its run-time address.  */

Elf32_Addr
_dl_symbol_value (struct link_map *map, const char *name)
{
  Elf32_Addr loadbase;
  const Elf32_Sym *ref = NULL;
  struct link_map *scope[2] = { map, NULL };
  loadbase = _dl_lookup_symbol (name, &ref, scope, map->l_name, 0, 0);
  return loadbase + ref->st_value;
}