nss_db: protect against empty mappings

[DJ Delorie <dj at redhat dot com>]

Resolves: #24696

(note to reviewers: Sergi contributed the one-line fix, I did the
rest, for the purposes of copyright-line-counts)

2019-06-17  DJ Delorie  <dj@redhat.com>
	    Sergei Trofimovich <slyfox@inbox.ru>

	* nss/nss_db/db-open.c (internal_endent): Protect against NULL
	mappings.
	* nss/tst-nss-db-endgrent.c: New.
	* nss/tst-nss-db-endgrent.root: New.
	* nss/Makefile: Add new test.

diff --git a/nss/Makefile b/nss/Makefile
index 15fc410cf1..a15c3b7d90 100644
--- a/nss/Makefile
+++ b/nss/Makefile
@@ -62,7 +62,8 @@ xtests			= bug-erange
 tests-container = \
 			  tst-nss-test3 \
 			  tst-nss-files-hosts-long \
-			  tst-nss-db-endpwent
+			  tst-nss-db-endpwent \
+			  tst-nss-db-endgrent
 
 # Tests which need libdl
 ifeq (yes,$(build-shared))
diff --git a/nss/nss_db/db-open.c b/nss/nss_db/db-open.c
index f7c53b4486..3fa11e9ab0 100644
--- a/nss/nss_db/db-open.c
+++ b/nss/nss_db/db-open.c
@@ -63,6 +63,9 @@ internal_setent (const char *file, struct nss_db_map *mapping)
 void
 internal_endent (struct nss_db_map *mapping)
 {
-  munmap (mapping->header, mapping->len);
-  mapping->header = NULL;
+  if (mapping->header != NULL)
+    {
+      munmap (mapping->header, mapping->len);
+      mapping->header = NULL;
+    }
 }
diff --git a/nss/tst-nss-db-endgrent.c b/nss/tst-nss-db-endgrent.c
new file mode 100644
index 0000000000..f4c1e14d60
--- /dev/null
+++ b/nss/tst-nss-db-endgrent.c
@@ -0,0 +1,46 @@
+/* Test for endgrent changing errno.
+   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 <stdlib.h>
+#include <sys/types.h>
+#include <grp.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include <support/check.h>
+#include <support/support.h>
+
+
+static int
+do_test (void)
+{
+  /* Just make sure it's not there, although usually it won't be.  */
+  unlink ("/var/db/group.db");
+
+  getgrent ();
+
+  int saved_errno = errno;
+
+  endgrent ();
+
+  if (errno != saved_errno)
+    FAIL_EXIT1 ("endgrent changed errno from %d o %d\n", saved_errno, errno);
+
+  return 0;
+}
+#include <support/test-driver.c>
diff --git a/nss/tst-nss-db-endgrent.root/etc/nsswitch.conf b/nss/tst-nss-db-endgrent.root/etc/nsswitch.conf
new file mode 100644
index 0000000000..21471df94f
--- /dev/null
+++ b/nss/tst-nss-db-endgrent.root/etc/nsswitch.conf
@@ -0,0 +1 @@
+group : db files