glibc/elf/do-lookup.h

/* Look up a symbol in the loaded objects.
   Copyright (C) 1995-2007, 2008, 2009 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 Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */


/* Inner part of the lookup functions.  We return a value > 0 if we
   found the symbol, the value 0 if nothing is found and < 0 if
   something bad happened.  */
static int
__attribute_noinline__
do_lookup_x (const char *undef_name, uint_fast32_t new_hash,
	     unsigned long int *old_hash, const ElfW(Sym) *ref,
	     struct sym_val *result, struct r_scope_elem *scope, size_t i,
	     const struct r_found_version *const version, int flags,
	     struct link_map *skip, int type_class)
{
  size_t n = scope->r_nlist;
  /* Make sure we read the value before proceeding.  Otherwise we
     might use r_list pointing to the initial scope and r_nlist being
     the value after a resize.  That is the only path in dl-open.c not
     protected by GSCOPE.  A read barrier here might be to expensive.  */
  __asm volatile ("" : "+r" (n), "+m" (scope->r_list));
  struct link_map **list = scope->r_list;

  do
    {
      /* These variables are used in the nested function.  */
      Elf_Symndx symidx;
      int num_versions = 0;
      const ElfW(Sym) *versioned_sym = NULL;

      const struct link_map *map = list[i]->l_real;

      /* Here come the extra test needed for `_dl_lookup_symbol_skip'.  */
      if (map == skip)
	continue;

      /* Don't search the executable when resolving a copy reloc.  */
      if ((type_class & ELF_RTYPE_CLASS_COPY) && map->l_type == lt_executable)
	continue;

      /* Do not look into objects which are going to be removed.  */
      if (map->l_removed)
	continue;

      /* Print some debugging info if wanted.  */
      if (__builtin_expect (GLRO(dl_debug_mask) & DL_DEBUG_SYMBOLS, 0))
	_dl_debug_printf ("symbol=%s;  lookup in file=%s [%lu]\n",
			  undef_name,
			  map->l_name[0] ? map->l_name : rtld_progname,
			  map->l_ns);

      /* If the hash table is empty there is nothing to do here.  */
      if (map->l_nbuckets == 0)
	continue;

      /* The tables for this map.  */
      const ElfW(Sym) *symtab = (const void *) D_PTR (map, l_info[DT_SYMTAB]);
      const char *strtab = (const void *) D_PTR (map, l_info[DT_STRTAB]);


      /* Nested routine to check whether the symbol matches.  */
      const ElfW(Sym) *
      __attribute_noinline__
      check_match (const ElfW(Sym) *sym)
      {
	unsigned int stt = ELFW(ST_TYPE) (sym->st_info);
	assert (ELF_RTYPE_CLASS_PLT == 1);
	if (__builtin_expect ((sym->st_value == 0 /* No value.  */
			       && stt != STT_TLS)
			      || (type_class & (sym->st_shndx == SHN_UNDEF)),
			      0))
	  return NULL;

	/* Ignore all but STT_NOTYPE, STT_OBJECT, STT_FUNC,
	   STT_COMMON, STT_TLS, and STT_GNU_IFUNC since these are no
	   code/data definitions.  */
#define ALLOWED_STT \
	((1 << STT_NOTYPE) | (1 << STT_OBJECT) | (1 << STT_FUNC) \
	 | (1 << STT_COMMON) | (1 << STT_TLS) | (1 << STT_GNU_IFUNC))
	if (__builtin_expect (((1 << stt) & ALLOWED_STT) == 0, 0))
	  return NULL;

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

	const ElfW(Half) *verstab = map->l_versyms;
	if (version != NULL)
	  {
	    if (__builtin_expect (verstab == NULL, 0))
	      {
		/* We need a versioned symbol but haven't found any.  If
		   this is the object which is referenced in the verneed
		   entry it is a bug in the library since a symbol must
		   not simply disappear.

		   It would also be a bug in the object since it means that
		   the list of required versions is incomplete and so the
		   tests in dl-version.c haven't found a problem.*/
		assert (version->filename == NULL
			|| ! _dl_name_match_p (version->filename, map));

		/* Otherwise we accept the symbol.  */
	      }
	    else
	      {
		/* We can match the version information or use the
		   default one if it is not hidden.  */
		ElfW(Half) ndx = verstab[symidx] & 0x7fff;
		if ((map->l_versions[ndx].hash != version->hash
		     || strcmp (map->l_versions[ndx].name, version->name))
		    && (version->hidden || map->l_versions[ndx].hash
			|| (verstab[symidx] & 0x8000)))
		  /* It's not the version we want.  */
		  return NULL;
	      }
	  }
	else
	  {
	    /* No specific version is selected.  There are two ways we
	       can got here:

	       - a binary which does not include versioning information
	       is loaded

	       - dlsym() instead of dlvsym() is used to get a symbol which
	       might exist in more than one form

	       If the library does not provide symbol version information
	       there is no problem at at: we simply use the symbol if it
	       is defined.

	       These two lookups need to be handled differently if the
	       library defines versions.  In the case of the old
	       unversioned application the oldest (default) version
	       should be used.  In case of a dlsym() call the latest and
	       public interface should be returned.  */
	    if (verstab != NULL)
	      {
		if ((verstab[symidx] & 0x7fff)
		    >= ((flags & DL_LOOKUP_RETURN_NEWEST) ? 2 : 3))
		  {
		    /* Don't accept hidden symbols.  */
		    if ((verstab[symidx] & 0x8000) == 0
			&& num_versions++ == 0)
		      /* No version so far.  */
		      versioned_sym = sym;

		    return NULL;
		  }
	      }
	  }

	/* There cannot be another entry for this symbol so stop here.  */
	return sym;
      }

      const ElfW(Sym) *sym;
      const ElfW(Addr) *bitmask = map->l_gnu_bitmask;
      if (__builtin_expect (bitmask != NULL, 1))
	{
	  ElfW(Addr) bitmask_word
	    = bitmask[(new_hash / __ELF_NATIVE_CLASS)
		      & map->l_gnu_bitmask_idxbits];

	  unsigned int hashbit1 = new_hash & (__ELF_NATIVE_CLASS - 1);
	  unsigned int hashbit2 = ((new_hash >> map->l_gnu_shift)
				   & (__ELF_NATIVE_CLASS - 1));

	  if (__builtin_expect ((bitmask_word >> hashbit1)
				& (bitmask_word >> hashbit2) & 1, 0))
	    {
	      Elf32_Word bucket = map->l_gnu_buckets[new_hash
						     % map->l_nbuckets];
	      if (bucket != 0)
		{
		  const Elf32_Word *hasharr = &map->l_gnu_chain_zero[bucket];

		  do
		    if (((*hasharr ^ new_hash) >> 1) == 0)
		      {
			symidx = hasharr - map->l_gnu_chain_zero;
			sym = check_match (&symtab[symidx]);
			if (sym != NULL)
			  goto found_it;
		      }
		  while ((*hasharr++ & 1u) == 0);
		}
	    }
	  /* No symbol found.  */
	  symidx = SHN_UNDEF;
	}
      else
	{
	  if (*old_hash == 0xffffffff)
	    *old_hash = _dl_elf_hash (undef_name);

	  /* Use the old SysV-style hash table.  Search the appropriate
	     hash bucket in this object's symbol table for a definition
	     for the same symbol name.  */
	  for (symidx = map->l_buckets[*old_hash % map->l_nbuckets];
	       symidx != STN_UNDEF;
	       symidx = map->l_chain[symidx])
	    {
	      sym = check_match (&symtab[symidx]);
	      if (sym != NULL)
		goto found_it;
	    }
	}

      /* If we have seen exactly one versioned symbol while we are
	 looking for an unversioned symbol and the version is not the
	 default version we still accept this symbol since there are
	 no possible ambiguities.  */
      sym = num_versions == 1 ? versioned_sym : NULL;

      if (sym != NULL)
	{
	found_it:
	  switch (ELFW(ST_BIND) (sym->st_info))
	    {
	    case STB_WEAK:
	      /* Weak definition.  Use this value if we don't find another.  */
	      if (__builtin_expect (GLRO(dl_dynamic_weak), 0))
		{
		  if (! result->s)
		    {
		      result->s = sym;
		      result->m = (struct link_map *) map;
		    }
		  break;
		}
	      /* FALLTHROUGH */
	    case STB_GLOBAL:
	      /* Global definition.  Just what we need.  */
	      result->s = sym;
	      result->m = (struct link_map *) map;
	      return 1;
	    default:
	      /* Local symbols are ignored.  */
	      break;
	    }
	}

      /* If this current map is the one mentioned in the verneed entry
	 and we have not found a weak entry, it is a bug.  */
      if (symidx == STN_UNDEF && version != NULL && version->filename != NULL
	  && __builtin_expect (_dl_name_match_p (version->filename, map), 0))
	return -1;
    }
  while (++i < n);

  /* We have not found anything until now.  */
  return 0;
}