getmntent: fix memory corruption w/blank lines [BZ #18887]

The fix for BZ #17273 introduced a single byte of memory corruption when
the line is entirely blank.  It would walk back past the start of the
buffer if the heap happened to be 0x20 or 0x09 and then write a NUL byte.
	buffer = '\n';
	end_ptr = buffer;
	while (end_ptr[-1] == ' ' || end_ptr[-1] == '\t')
		end_ptr--;
	*end_ptr = '\0';

Fix that and rework the tests.  Adding the testcase for BZ #17273 to the
existing \040 parser does not really make sense as it's unrelated, and
leads to confusing behavior: it implicitly relies on the new entry being
longer than the previous entry (since it just rewinds the FILE*).  Split
it out into its own dedicated testcase instead.

(cherry picked from commit b0e805fa0d6fea33745952df7b7f5442ca4c374f)

5 files changed, 103 insertions, 22 deletions

diff --git a/misc/Makefile b/misc/Makefile
index aecb0dae9d..2f5edf6316 100644
--- a/misc/Makefile
+++ b/misc/Makefile
@@ -76,7 +76,8 @@ install-lib := libg.a
 gpl2lgpl := error.c error.h
 
 tests := tst-dirname tst-tsearch tst-fdset tst-efgcvt tst-mntent tst-hsearch \
-	 tst-error1 tst-pselect tst-insremque tst-mntent2 bug-hsearch1
+	 tst-error1 tst-pselect tst-insremque tst-mntent2 bug-hsearch1 \
+	 tst-mntent-blank-corrupt tst-mntent-blank-passno
 ifeq ($(run-built-tests),yes)
 tests-special += $(objpfx)tst-error1-mem.out
 endif
diff --git a/misc/mntent_r.c b/misc/mntent_r.c
index 615987347a..4f269984d7 100644
--- a/misc/mntent_r.c
+++ b/misc/mntent_r.c
@@ -136,7 +136,9 @@ __getmntent_r (FILE *stream, struct mntent *mp, char *buffer, int bufsiz)
       end_ptr = strchr (buffer, '\n');
       if (end_ptr != NULL)	/* chop newline */
 	{
-	  while (end_ptr[-1] == ' ' || end_ptr[-1] == '\t')
+	  /* Do not walk past the start of buffer if it's all whitespace.  */
+	  while (end_ptr != buffer
+		 && (end_ptr[-1] == ' ' || end_ptr[-1] == '\t'))
             end_ptr--;
 	  *end_ptr = '\0';
 	}
diff --git a/misc/tst-mntent-blank-corrupt.c b/misc/tst-mntent-blank-corrupt.c
new file mode 100644
index 0000000000..92266a35b5
--- /dev/null
+++ b/misc/tst-mntent-blank-corrupt.c
@@ -0,0 +1,45 @@
+/* Make sure blank lines does not cause memory corruption BZ #18887.
+
+   Copyright (C) 2009-2015 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 <mntent.h>
+#include <stdio.h>
+#include <string.h>
+
+/* Make sure blank lines don't trigger memory corruption.  This doesn't happen
+   for all targets though, so it's a best effort test BZ #18887.  */
+static int
+do_test (void)
+{
+  FILE *fp;
+
+  fp = tmpfile ();
+  fputs ("\n \n/foo\\040dir /bar\\040dir auto bind \t \n", fp);
+  rewind (fp);
+
+  /* The corruption happens here ...  */
+  getmntent (fp);
+  /* ... but trigers here.  */
+  endmntent (fp);
+
+  /* If the test failed, we would crash, and not hit this point.  */
+  return 0;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/misc/tst-mntent-blank-passno.c b/misc/tst-mntent-blank-passno.c
new file mode 100644
index 0000000000..fc0429197f
--- /dev/null
+++ b/misc/tst-mntent-blank-passno.c
@@ -0,0 +1,53 @@
+/* Make sure trailing whitespace is handled properly BZ #17273.
+
+   Copyright (C) 2009-2015 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 <mntent.h>
+#include <stdio.h>
+#include <string.h>
+
+/* Check entries to make sure trailing whitespace is ignored and we return the
+   correct passno value BZ #17273.  */
+static int
+do_test (void)
+{
+  int result = 0;
+  FILE *fp;
+  struct mntent *mnt;
+
+  fp = tmpfile ();
+  fputs ("/foo\\040dir /bar\\040dir auto bind \t \n", fp);
+  rewind (fp);
+
+  mnt = getmntent (fp);
+  if (strcmp (mnt->mnt_fsname, "/foo dir") != 0
+      || strcmp (mnt->mnt_dir, "/bar dir") != 0
+      || strcmp (mnt->mnt_type, "auto") != 0
+      || strcmp (mnt->mnt_opts, "bind") != 0
+      || mnt->mnt_freq != 0
+      || mnt->mnt_passno != 0)
+    {
+      puts ("Error while reading entry with trailing whitespaces");
+      result = 1;
+    }
+
+  return result;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/misc/tst-mntent.c b/misc/tst-mntent.c
index 876c89f8ed..820b35493b 100644
--- a/misc/tst-mntent.c
+++ b/misc/tst-mntent.c
@@ -73,26 +73,6 @@ main (int argc, char *argv[])
 	  puts ("Error while reading written entry back in");
 	  result = 1;
 	}
-
-      /* Part III: Entry with whitespaces at the end of a line. */
-      rewind (fp);
-
-      fputs ("/foo\\040dir /bar\\040dir auto bind \t \n", fp);
-
-      rewind (fp);
-
-      mnt = getmntent (fp);
-
-      if (strcmp (mnt->mnt_fsname, "/foo dir") != 0
-	  || strcmp (mnt->mnt_dir, "/bar dir") != 0
-	  || strcmp (mnt->mnt_type, "auto") != 0
-	  || strcmp (mnt->mnt_opts, "bind") != 0
-	  || mnt->mnt_freq != 0
-	  || mnt->mnt_passno != 0)
-	{
-	  puts ("Error while reading entry with trailing whitespaces");
-	  result = 1;
-	}
    }
 
   return result;