dlsym: Do not determine caller link map if not needed

Obtaining the link map is potentially very slow because it requires
iterating over all loaded objects in the current implementation.  If
the caller supplied an explicit handle (i.e., not one of the RTLD_*
constants), the dlsym implementation does not need the identity of the
caller (except in the special case of auditing), so this change
avoids computing it in that case.

Even in the minimal case (dlsym called from a main program linked with
-dl), this shows a small speedup, perhaps around five percent.  The
performance improvement can be arbitrarily large in principle (if
_dl_find_dso_for_object has to iterate over many link maps).

Change-Id: Ide5d9e2cc7ac25a0ffae8fb4c26def0c898efa29

1 files changed, 21 insertions, 4 deletions

diff --git a/elf/dl-sym.c b/elf/dl-sym.c
index 21e025b..3933194 100644
--- a/elf/dl-sym.c
+++ b/elf/dl-sym.c
@@ -80,6 +80,18 @@ call_dl_lookup (void *ptr)
 					args->flags, NULL);
 }
 
+/* Return the link map containing the caller address.  */
+static inline struct link_map *
+find_caller_link_map (ElfW(Addr) caller)
+{
+  struct link_map *l = _dl_find_dso_for_object (caller);
+  if (l != NULL)
+    return l;
+  else
+    /* If the address is not recognized the call comes from the main
+       program (we hope).  */
+    return GL(dl_ns)[LM_ID_BASE]._ns_loaded;
+}
 
 static void *
 do_sym (void *handle, const char *name, void *who,
@@ -89,13 +101,13 @@ do_sym (void *handle, const char *name, void *who,
   lookup_t result;
   ElfW(Addr) caller = (ElfW(Addr)) who;
 
-  struct link_map *l = _dl_find_dso_for_object (caller);
-  /* If the address is not recognized the call comes from the main
-     program (we hope).  */
-  struct link_map *match = l ? l : GL(dl_ns)[LM_ID_BASE]._ns_loaded;
+  /* Link map of the caller if needed.  */
+  struct link_map *match = NULL;
 
   if (handle == RTLD_DEFAULT)
     {
+      match = find_caller_link_map (caller);
+
       /* Search the global scope.  We have the simple case where
 	 we look up in the scope of an object which was part of
 	 the initial binary.  And then the more complex part
@@ -128,6 +140,8 @@ do_sym (void *handle, const char *name, void *who,
     }
   else if (handle == RTLD_NEXT)
     {
+      match = find_caller_link_map (caller);
+
       if (__glibc_unlikely (match == GL(dl_ns)[LM_ID_BASE]._ns_loaded))
 	{
 	  if (match == NULL
@@ -187,6 +201,9 @@ RTLD_NEXT used in code not dynamically loaded"));
 	  unsigned int ndx = (ref - (ElfW(Sym) *) D_PTR (result,
 							 l_info[DT_SYMTAB]));
 
+	  if (match == NULL)
+	    match = find_caller_link_map (caller);
+
 	  if ((match->l_audit_any_plt | result->l_audit_any_plt) != 0)
 	    {
 	      unsigned int altvalue = 0;