Test that errno is set to 0 at program startup

Add new testcase elf/tst-startup-errno.c which tests that errno is set
to 0 at first ELF constructor execution and at the start of the
program's main function.

Tested for x86_64

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

2 files changed, 59 insertions, 0 deletions

diff --git a/elf/Makefile b/elf/Makefile
index 9ea4baf323..09d77093a7 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -457,6 +457,7 @@ tests += \
   tst-single_threaded-pthread \
   tst-sonamemove-dlopen \
   tst-sonamemove-link \
+  tst-startup-errno \
   tst-thrlock \
   tst-tls-dlinfo \
   tst-tls-ie \
diff --git a/elf/tst-startup-errno.c b/elf/tst-startup-errno.c
new file mode 100644
index 0000000000..59a1005fb6
--- /dev/null
+++ b/elf/tst-startup-errno.c
@@ -0,0 +1,58 @@
+/* Test the value of errno at program startup.
+   Copyright (C) 2024 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 <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+/* Verify that errno is 0 at first ELF constructor execution and at
+   the start of main.  */
+
+static void set_ctor_errno (void) __attribute__((constructor));
+static int ctor_errno = -1;
+
+static void
+set_ctor_errno (void)
+{
+  ctor_errno = errno;
+}
+
+static int
+get_ctor_errno (void)
+{
+  return ctor_errno;
+}
+
+int
+main (void)
+{
+  if (errno != 0)
+    {
+      printf ("At start of main errno set to %d != 0\n", errno);
+      exit (1);
+    }
+
+  if (get_ctor_errno () != 0)
+    {
+      printf ("At ctor exec errno set to %d != 0\n", get_ctor_errno ());
+      exit (1);
+    }
+
+  return 0;
+}
+