[PATCH v2 2/6] elf: Avoid unnecessary slowdown from profiling with audit (BZ#15533)

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

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 (la_plt{enter,exit}
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.

Co-authored-by: Alexander Monakov <amonakov@ispras.dot.ru>
---
 NEWS                 |  4 ++++
 elf/Makefile         |  6 +++++-
 elf/dl-reloc.c       | 12 +++++++++++-
 elf/rtld.c           |  8 +-------
 elf/tst-audit18.c    | 39 +++++++++++++++++++++++++++++++++++++++
 elf/tst-auditmod18.c | 24 ++++++++++++++++++++++++
 include/link.h       |  2 ++
 7 files changed, 86 insertions(+), 9 deletions(-)
 create mode 100644 elf/tst-audit18.c
 create mode 100644 elf/tst-auditmod18.c

diff --git a/NEWS b/NEWS
index 13ffe627da..8fde312ec6 100644
--- a/NEWS
+++ b/NEWS
@@ -76,6 +76,10 @@ Major new features:
   equal to a giver integer.  This function is a GNU extension, although
   Solaris also provides a similar function.
 
+* The audit libraries will avoid unnecessary slowdown if it is not required
+  either PLT tracking or symbol binding profiling (enabled with LA_FLG_BINDFROM
+  or LA_FLG_BINDTO from la_objopen() callback).
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The function pthread_mutex_consistent_np has been deprecated; programs
diff --git a/elf/Makefile b/elf/Makefile
index 3216d67bb4..9e2d766c03 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -219,7 +219,7 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-dlopen-self tst-auditmany tst-initfinilazyfail tst-dlopenfail \
 	 tst-dlopenfail-2 \
 	 tst-filterobj tst-filterobj-dlopen tst-auxobj tst-auxobj-dlopen \
-	 tst-audit14 tst-audit15 tst-audit16 tst-audit17 \
+	 tst-audit14 tst-audit15 tst-audit16 tst-audit17 tst-audit18 \
 	 tst-single_threaded tst-single_threaded-pthread \
 	 tst-tls-ie tst-tls-ie-dlmopen argv0test \
 	 tst-glibc-hwcaps tst-glibc-hwcaps-prepend tst-glibc-hwcaps-mask \
@@ -301,6 +301,7 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
 		tst-unique1mod1 tst-unique1mod2 \
 		tst-unique2mod1 tst-unique2mod2 \
 		tst-auditmod9a tst-auditmod9b \
+		tst-auditmod18 \
 		$(if $(CXX),tst-unique3lib tst-unique3lib2 tst-unique4lib \
 		  tst-nodelete-uniquemod tst-nodelete-rtldmod \
 		  tst-nodelete-zmod \
@@ -1489,6 +1490,9 @@ $(objpfx)tst-auditmod17.so: $(objpfx)tst-auditmod17.os
 CFLAGS-.os += $(call elide-stack-protector,.os,tst-auditmod17)
 tst-audit17-ENV = LD_AUDIT=$(objpfx)tst-auditmod17.so
 
+$(objpfx)tst-audit18.out: $(objpfx)tst-auditmod18.so
+tst-audit18-ENV = LD_AUDIT=$(objpfx)tst-auditmod18.so
+
 # tst-sonamemove links against an older implementation of the library.
 LDFLAGS-tst-sonamemove-linkmod1.so = \
   -Wl,--version-script=tst-sonamemove-linkmod1.map \
diff --git a/elf/dl-reloc.c b/elf/dl-reloc.c
index e13a672ade..2abcfc996f 100644
--- a/elf/dl-reloc.c
+++ b/elf/dl-reloc.c
@@ -181,7 +181,17 @@ _dl_relocate_object (struct link_map *l, struct r_scope_elem *scope[],
 #ifdef SHARED
   /* If we are auditing, install the same handlers we need for profiling.  */
   if ((reloc_mode & __RTLD_AUDIT) == 0)
-    consider_profiling |= GLRO(dl_audit) != NULL;
+    {
+      struct audit_ifaces *afct = GLRO(dl_audit);
+      for (unsigned int cnt = 0; cnt < GLRO(dl_naudit); ++cnt)
+	{
+	  /* Profiling is needed only if PLT hooks are provided.  */
+	  if (afct->ARCH_LA_PLTENTER != NULL
+	      || afct->ARCH_LA_PLTEXIT != NULL)
+	    consider_profiling = 1;
+	  afct = afct->next;
+	}
+    }
 #elif defined PROF
   /* Never use dynamic linker profiling for gprof profiling code.  */
 # define consider_profiling 0
diff --git a/elf/rtld.c b/elf/rtld.c
index d733359eaf..374bf86a69 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -1013,13 +1013,7 @@ ERROR: audit interface '%s' requires version %d (maximum supported version %d);
     "la_objsearch\0"
     "la_objopen\0"
     "la_preinit\0"
-#if __ELF_NATIVE_CLASS == 32
-    "la_symbind32\0"
-#elif __ELF_NATIVE_CLASS == 64
-    "la_symbind64\0"
-#else
-# error "__ELF_NATIVE_CLASS must be defined"
-#endif
+    LA_SYMBIND "\0"
 #define STRING(s) __STRING (s)
     "la_" STRING (ARCH_LA_PLTENTER) "\0"
     "la_" STRING (ARCH_LA_PLTEXIT) "\0"
diff --git a/elf/tst-audit18.c b/elf/tst-audit18.c
new file mode 100644
index 0000000000..36b781f9be
--- /dev/null
+++ b/elf/tst-audit18.c
@@ -0,0 +1,39 @@
+/* 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 <stdio.h>
+
+/* We interpose the profile resolver and if it is called it means profiling is
+   enabled.  */
+void
+_dl_runtime_profile (ElfW(Word) addr)
+{
+  volatile int *p = NULL;
+  *p = 0;
+}
+
+static int
+do_test (void)
+{
+  printf ("...");
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-auditmod18.c b/elf/tst-auditmod18.c
new file mode 100644
index 0000000000..6d1b55223e
--- /dev/null
+++ b/elf/tst-auditmod18.c
@@ -0,0 +1,24 @@
+/* Check if DT_ADIT 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/>.  */
+
+unsigned int
+la_version (unsigned int version)
+{
+  return version;
+}
diff --git a/include/link.h b/include/link.h
index 4af16cb596..ebd0f511e2 100644
--- a/include/link.h
+++ b/include/link.h
@@ -355,8 +355,10 @@ struct auditstate
 
 #if __ELF_NATIVE_CLASS == 32
 # define symbind symbind32
+# define LA_SYMBIND "la_symbind32"
 #elif __ELF_NATIVE_CLASS == 64
 # define symbind symbind64
+# define LA_SYMBIND "la_symbind64"
 #else
 # error "__ELF_NATIVE_CLASS must be defined"
 #endif
-- 
2.30.2