diff mbox series

[review,v2] dlsym: Do not determine caller link map if not needed

Message ID 20191127185810.4CD3820AF6@gnutoolchain-gerrit.osci.io
State New
Headers show
Series [review,v2] dlsym: Do not determine caller link map if not needed | expand

Commit Message

Adhemerval Zanella (Code Review) Nov. 27, 2019, 6:58 p.m. UTC 
Change URL: https://gnutoolchain-gerrit.osci.io/r/c/glibc/+/528
......................................................................

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
---
M elf/dl-sym.c
1 file changed, 21 insertions(+), 4 deletions(-)
diff mbox series

Patch

diff --git a/elf/dl-sym.c b/elf/dl-sym.c
index 8209342..7282e09 100644
--- a/elf/dl-sym.c
+++ b/elf/dl-sym.c
@@ -80,6 +80,18 @@ 
 					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 @@ 
   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 @@ 
     }
   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 @@ 
 	  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;