Avoid ELF lookup race.

On some architectures the update of the l_used field in the lookup
functions races with setting the other bits in the bitfield.  Simply
avoid this and optimize use of l_used in general.

diff --git a/ChangeLog b/ChangeLog
index ce4d4e8..b1e14b6 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2009-12-15  Ulrich Drepper  <drepper@redhat.com>
+
+       * include/link.h (struct link_map): Move l_used into its own word.
+       * elf/dl-lookup.c (_dl_lookup_symbol_x): Only update l_used when it is
+       still zero.
+       * elf/dl-object.c (_dl_new_object): Set dl_used if we know it is
+       never really used.
+
 2009-12-13  H.J. Lu  <hongjiu.lu@intel.com>
 
        * sysdeps/i386/i686/multiarch/strcspn.S Include <init-arch.h>

diff --git a/elf/dl-lookup.c b/elf/dl-lookup.c
index c1a1366..763ec16 100644 (file)
--- a/elf/dl-lookup.c
+++ b/elf/dl-lookup.c
@@ -777,7 +777,7 @@ _dl_lookup_symbol_x (const char *undef_name, struct link_map *undef_map,
   if (__builtin_expect (protected != 0, 0))
     {
       /* It is very tricky.  We need to figure out what value to
-         return for the protected symbol.  */
+        return for the protected symbol.  */
       if (type_class == ELF_RTYPE_CLASS_PLT)
        {
          if (current_value.s != NULL && current_value.m != undef_map)
@@ -822,7 +822,8 @@ _dl_lookup_symbol_x (const char *undef_name, struct link_map *undef_map,
                                  version, type_class, flags, skip_map);
 
   /* The object is used.  */
-  current_value.m->l_used = 1;
+  if (__builtin_expect (current_value.m->l_used == 0, 0))
+    current_value.m->l_used = 1;
 
   if (__builtin_expect (GLRO(dl_debug_mask)
                        & (DL_DEBUG_BINDINGS|DL_DEBUG_PRELINK), 0))
@@ -844,7 +845,7 @@ _dl_setup_hash (struct link_map *map)
   Elf_Symndx nchain;
 
   if (__builtin_expect (map->l_info[DT_ADDRTAGIDX (DT_GNU_HASH) + DT_NUM
-                                   + DT_THISPROCNUM + DT_VERSIONTAGNUM
+                                   + DT_THISPROCNUM + DT_VERSIONTAGNUM
                                    + DT_EXTRANUM + DT_VALNUM] != NULL, 1))
     {
       Elf32_Word *hash32

diff --git a/elf/dl-object.c b/elf/dl-object.c
index be4ea38..788e2c0 100644 (file)
--- a/elf/dl-object.c
+++ b/elf/dl-object.c
@@ -1,5 +1,5 @@
 /* Storage management for the chain of loaded shared objects.
-   Copyright (C) 1995-2002,2004,2006,2007,2008 Free Software Foundation, Inc.
+   Copyright (C) 1995-2002,2004,2006-2008,2009 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
@@ -67,6 +67,10 @@ _dl_new_object (char *realname, const char *libname, int type,
 
   new->l_name = realname;
   new->l_type = type;
+  /* If we set the bit now since we know it is never used we avoid
+     dirtying the cache line later.  */
+  if ((GLRO(dl_debug_mask) & DL_DEBUG_UNUSED) == 0)
+    new->l_used = 1;
   new->l_loader = loader;
 #if NO_TLS_OFFSET != 0
   new->l_tls_offset = NO_TLS_OFFSET;
@@ -174,7 +178,7 @@ _dl_new_object (char *realname, const char *libname, int type,
          if (result == NULL)
            {
              /* We were not able to determine the current directory.
-                Note that free(origin) is OK if origin == NULL.  */
+                Note that free(origin) is OK if origin == NULL.  */
              free (origin);
              origin = (char *) -1;
              goto out;

diff --git a/include/link.h b/include/link.h
index 4b9978a..26c6743 100644 (file)
--- a/include/link.h
+++ b/include/link.h
@@ -1,6 +1,6 @@
 /* Data structure for communication from the run-time dynamic linker for
    loaded ELF shared objects.
-   Copyright (C) 1995-2006, 2007 Free Software Foundation, Inc.
+   Copyright (C) 1995-2006, 2007, 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
@@ -180,7 +180,6 @@ struct link_map
     unsigned int l_need_tls_init:1; /* Nonzero if GL(dl_init_static_tls)
                                       should be called on this link map
                                       when relocation finishes.  */
-    unsigned int l_used:1;     /* Nonzero if the DSO is used.  */
     unsigned int l_auditing:1; /* Nonzero if the DSO is used in auditing.  */
     unsigned int l_audit_any_plt:1; /* Nonzero if at least one audit module
                                       is interested in the PLT interception.*/
@@ -239,12 +238,15 @@ struct link_map
     struct link_map **l_initfini;
 
     /* List of the dependencies introduced through symbol binding.  */
-    unsigned int l_reldepsmax;
     struct link_map_reldeps
       {
        unsigned int act;
        struct link_map *list[];
       } *l_reldeps;
+    unsigned int l_reldepsmax;
+
+    /* Nonzero if the DSO is used.  */
+    unsigned int l_used;
 
     /* Various flag words.  */
     ElfW(Word) l_feature_1;