elf: Avoid unnecessary slowdown from profiling with audit (BZ#15533)

The rtld-audit interfaces introduces a slowdown due to enabling
profiling instrumentation (as if LD_AUDIT implied LD_PROFILE).
However, instrumenting is only necessary if one of audit libraries
provides PLT callbacks (la_pltenter or la_pltexit symbols).  Otherwise,
the slowdown can be avoided.

The following patch adjusts the logic that enables profiling to iterate
over all audit modules and check if any of those provides a PLT hook.
To keep la_symbind to work even without PLT callbacks, _dl_fixup now
calls the audit callback if the modules implements it.

Co-authored-by: Alexander Monakov <amonakov@ispras.ru>

Checked on x86_64-linux-gnu, i686-linux-gnu, and aarch64-linux-gnu.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

1 files changed, 31 insertions, 0 deletions

diff --git a/elf/dl-runtime.c b/elf/dl-runtime.c
index e42f6e8..77a5ccc 100644
--- a/elf/dl-runtime.c
+++ b/elf/dl-runtime.c
@@ -124,6 +124,37 @@ _dl_fixup (
       && __builtin_expect (ELFW(ST_TYPE) (sym->st_info) == STT_GNU_IFUNC, 0))
     value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value));
 
+#ifdef SHARED
+  /* Auditing checkpoint: we have a new binding.  Provide the auditing
+     libraries the possibility to change the value and tell us whether further
+     auditing is wanted.
+     The l_reloc_result is only allocated if there is an audit module which
+     provides a la_symbind.  */
+  if (l->l_reloc_result != NULL)
+    {
+      /* This is the address in the array where we store the result of previous
+	 relocations.  */
+      struct reloc_result *reloc_result
+	= &l->l_reloc_result[reloc_index (pltgot, reloc_arg, sizeof (PLTREL))];
+      unsigned int init = atomic_load_acquire (&reloc_result->init);
+      if (init == 0)
+	{
+	  _dl_audit_symbind (l, reloc_result, sym, &value, result);
+
+	  /* Store the result for later runs.  */
+	  if (__glibc_likely (! GLRO(dl_bind_not)))
+	    {
+	      reloc_result->addr = value;
+	      /* Guarantee all previous writes complete before init is
+		 updated.  See CONCURRENCY NOTES below.  */
+	      atomic_store_release (&reloc_result->init, 1);
+	    }
+	}
+      else
+	value = reloc_result->addr;
+    }
+#endif
+
   /* Finally, fix up the plt itself.  */
   if (__glibc_unlikely (GLRO(dl_bind_not)))
     return value;