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, 38 insertions, 0 deletions

diff --git a/elf/tst-audit19a.c b/elf/tst-audit19a.c
new file mode 100644
index 0000000000..035cde9351
--- /dev/null
+++ b/elf/tst-audit19a.c
@@ -0,0 +1,38 @@
+/* Check if DT_AUDIT a module without la_plt{enter,exit} symbols does not incur
+   in profiling (BZ#15533).
+   Copyright (C) 2021 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, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <link.h>
+#include <support/xdlfcn.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  void *h = xdlopen ("tst-auditmod19a.so", RTLD_NOW);
+
+  struct link_map *lmap;
+  TEST_VERIFY_EXIT (dlinfo (h, RTLD_DI_LINKMAP, &lmap) == 0);
+
+  /* The internal array is only allocated if profiling is enabled.  */
+  TEST_VERIFY (lmap->l_reloc_result == NULL);
+
+  return 0;
+}
+
+#include <support/test-driver.c>