debug: Improve '%n' fortify detection (BZ 30932)

The 7bb8045ec0 path made the '%n' fortify check ignore EMFILE errors
while trying to open /proc/self/maps, and this added a security
issue where EMFILE can be attacker-controlled thus making it
ineffective for some cases.

The EMFILE failure is reinstated but with a different error
message.  Also, to improve the false positive of the hardening for
the cases where no new files can be opened, the
_dl_readonly_area now uses  _dl_find_object to check if the
memory area is within a writable ELF segment.  The procfs method is
still used as fallback.

Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Arjun Shankar <arjun@redhat.com>

1 files changed, 56 insertions, 0 deletions

diff --git a/debug/tst-sprintf-fortify-rdonly-mod.c b/debug/tst-sprintf-fortify-rdonly-mod.c
new file mode 100644
index 0000000000..3655f27b32
--- /dev/null
+++ b/debug/tst-sprintf-fortify-rdonly-mod.c
@@ -0,0 +1,56 @@
+/* Testcase for BZ 30932.
+   Copyright (C) 2025 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 <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+static const char *str2 = "F";
+static char writeable_format[10] = "%s";
+static char relro_format[10] __attribute__ ((section (".data.rel.ro"))) =
+  "%s%n%s%n";
+
+void
+init_writable (void)
+{
+  strcpy (writeable_format + 2, "%n%s%n");
+}
+
+int
+sprintf_writable (int *n1, int *n2)
+{
+  char buf[128];
+  return sprintf (buf, writeable_format, str2, n1, str2, n2);
+}
+
+int
+sprintf_relro (int *n1, int *n2)
+{
+  char buf[128];
+  return sprintf (buf, relro_format, str2, n1, str2, n2);
+}
+
+int
+sprintf_writable_malloc (int *n1, int *n2)
+{
+  char buf[128];
+  char *buf2_malloc = strdup (writeable_format);
+  if (buf2_malloc == NULL)
+    abort ();
+  return sprintf (buf, buf2_malloc, str2, n1, str2, n2);
+}