Only build libm with -fno-math-errno (bug 24024)

Commit 1294b1892e ("Add support for sqrt asm redirects") added the
-fno-math-errno flag to build most of the glibc in order to enable GCC
to inline math functions. Due to GCC bug #88576, saving and restoring
errno around calls to malloc are optimized-out. In turn this causes
strerror to set errno to ENOMEM if it get passed an invalid error number
and if malloc sets errno to ENOMEM (which might happen even if it
succeeds). This is not allowed by POSIX.

This patch changes the build flags, building only libm with
-fno-math-errno and all the remaining code with -fno-math-errno. This
should be safe as libm doesn't contain any code saving and restoring
errno around malloc. This patch can probably be reverted once the GCC
bug is fixed and available in stable releases.

Tested on x86-64, no regression in the testsuite.

Changelog:
	[BZ #24024]
	* Makeconfig: Build libm with -fno-math-errno but build the remaining
	code with -fmath-errno.
	* string/Makefile [$(build-shared)] (tests): Add test-strerror-errno.
	[$(build-shared)] (LDLIBS-test-strerror-errno): New variable.
	* string/test-strerror-errno.c: New file.

2 files changed, 67 insertions, 0 deletions

diff --git a/string/Makefile b/string/Makefile
index bf4fb89..38b26a0 100644
--- a/string/Makefile
+++ b/string/Makefile
@@ -64,6 +64,12 @@ tests		:= tester inl-tester noinl-tester testcopy test-ffs	\
 # This test allocates a lot of memory and can run for a long time.
 xtests = tst-strcoll-overflow
 
+# This test needs libdl.
+ifeq (yes,$(build-shared))
+tests += test-strerror-errno
+LDLIBS-test-strerror-errno = $(libdl)
+endif
+
 ifeq ($(run-built-tests),yes)
 tests-special += $(objpfx)tst-svc-cmp.out
 endif
diff --git a/string/test-strerror-errno.c b/string/test-strerror-errno.c
new file mode 100644
index 0000000..8e744e7
--- /dev/null
+++ b/string/test-strerror-errno.c
@@ -0,0 +1,61 @@
+/* BZ #24024 strerror and errno test.
+
+   Copyright (C) 2019 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <dlfcn.h>
+#include <errno.h>
+#include <string.h>
+
+#include <support/check.h>
+#include <support/support.h>
+
+/* malloc is allowed to change errno to a value different than 0, even when
+   there is no actual error.  This happens for example when the memory
+   allocation through sbrk fails.  Simulate this by interposing our own
+   malloc implementation which sets errno to ENOMEM and calls the original
+   malloc.  */
+void
+*malloc (size_t size)
+{
+  static void *(*real_malloc) (size_t size);
+
+  if (!real_malloc)
+    real_malloc = dlsym (RTLD_NEXT, "malloc");
+
+  errno = ENOMEM;
+
+  return (*real_malloc) (size);
+}
+
+/* strerror must not change the value of errno.  Unfortunately due to GCC bug
+   #88576, this happens when -fmath-errno is used.  This simple test checks
+   that it doesn't happen.  */
+static int
+do_test (void)
+{
+  char *msg;
+
+  errno = 0;
+  msg = strerror (-3);
+  (void) msg;
+  TEST_COMPARE (errno, 0);
+
+  return 0;
+}
+
+#include <support/test-driver.c>