[2.28 COMMITTED] nss_db: fix endent wrt NULL mappings [BZ #24695] [BZ #24696]

[DJ Delorie <dj@redhat.com>]

From: DJ Delorie <dj@redhat.com>

nss_db allows for getpwent et al to be called without a set*ent,
but it only works once.  After the last get*ent a set*ent is
required to restart, because the end*ent did not properly reset
the module.  Resetting it to NULL allows for a proper restart.

If the database doesn't exist, however, end*ent erroniously called
munmap which set errno.

The test case runs "makedb" inside the testroot, so needs selinux
DSOs installed.

(cherry picked from commit 99135114ba23c3110b7e4e650fabdc5e639746b7)
(note: tests excluded as test-in-container infrastructure missing)

diff --git a/ChangeLog b/ChangeLog
index fcc0f8041d..d932485496 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2019-07-10  DJ Delorie  <dj@redhat.com>
+	    Sergei Trofimovich <slyfox@inbox.ru>
+
+	[BZ #24696]
+	[BZ #24695]
+	* nss/nss_db/db-open.c (internal_endent): Protect against NULL
+	mappings.
+
 2019-07-01  H.J. Lu  <hongjiu.lu@intel.com>
 
 	[BZ #24259]
diff --git a/nss/nss_db/db-open.c b/nss/nss_db/db-open.c
index 8538f8e961..ac430f445a 100644
--- a/nss/nss_db/db-open.c
+++ b/nss/nss_db/db-open.c
@@ -63,5 +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);
+  if (mapping->header != NULL)
+    {
+      munmap (mapping->header, mapping->len);
+      mapping->header = NULL;
+    }
 }