[v5] Only build libm with -fno-math-errno (bug 24024)

diff --git a/Makeconfig b/Makeconfig
index 9dc1a6ba8e..07007c9459 100644
--- a/Makeconfig
+++ b/Makeconfig
@@ -831,8 +831,10 @@  endif
 # disable any optimization that assume default rounding mode.
 +math-flags = -frounding-math
 
-# Build libc/libm using -fno-math-errno, but run testsuite with -fmath-errno.
-+extra-math-flags = $(if $(filter libnldbl nonlib testsuite,$(in-module)),-fmath-errno,-fno-math-errno)
+# Logically only "libnldbl", "nonlib" and "testsuite" should be using
+# -fno-math-errno. However due to GCC bug #88576, only "libm" can use
+# -fno-math-errno.
++extra-math-flags = $(if $(filter libm,$(in-module)),-fno-math-errno,-fmath-errno)
 
 # We might want to compile with some stack-protection flag.
 ifneq ($(stack-protector),)
diff --git a/string/Makefile b/string/Makefile
index bf4fb89b63..38b26a0f8e 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 0000000000..8e744e7ed9
--- /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>