devpts: correctly set default options

devpts_get_sb() calls memset(0) to clear mount options and calls
parse_mount_options() if user specified any mount options.

The memset(0) is bogus since the 'mode' and 'ptmxmode' options are
non-zero by default.  parse_mount_options() restores options to default
anyway and can properly deal with NULL mount options.

So in devpts_get_sb() remove memset(0) and call parse_mount_options() even
for NULL mount options.

Bug reported by Eric Paris: http://lkml.org/lkml/2009/5/7/448.
Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Reported-by: Eric Paris <eparis@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Reviewed-by: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/devpts/inode.c

@@ -90,6 +90,15 @@ static inline struct super_block *pts_sb_from_inode(struct inode *inode)
 #define PARSE_MOUNT	0
 #define PARSE_REMOUNT	1
 
+/*
+ * parse_mount_options():
+ * 	Set @opts to mount options specified in @data. If an option is not
+ * 	specified in @data, set it to its default value. The exception is
+ * 	'newinstance' option which can only be set/cleared on a mount (i.e.
+ * 	cannot be changed during remount).
+ *
+ * Note: @data may be NULL (in which case all options are set to default).
+ */
 static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
 {
 	char *p;
@@ -355,12 +364,9 @@ static int devpts_get_sb(struct file_system_type *fs_type,
 	struct pts_mount_opts opts;
 	struct super_block *s;
 
-	memset(&opts, 0, sizeof(opts));
-	if (data) {
 	error = parse_mount_options(data, PARSE_MOUNT, &opts);
 	if (error)
 		return error;
-	}
 
 	if (opts.newinstance)
 		s = sget(fs_type, NULL, set_anon_super, NULL);