ext4: Fix optional-arg mount options

We have 2 mount options, "barrier" and "auto_da_alloc" which may or
may not take a 1/0 argument.  This causes the ext4 superblock mount
code to subtract uninitialized pointers and pass the result to
kmalloc, which results in very noisy failures.

Per Ted's suggestion, initialize the args struct so that
we know whether match_token() found an argument for the
option, and skip match_int() if not.

Also, return error (0) from parse_options if we thought
we found an argument, but match_int() Fails.
Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -1229,6 +1229,11 @@ static int parse_options(char *options, struct super_block *sb,
 		if (!*p)
 			continue;
 
+		/*
+		 * Initialize args struct so we know whether arg was
+		 * found; some options take optional arguments.
+		 */
+		args[0].to = args[0].from = 0;
 		token = match_token(p, tokens, args);
 		switch (token) {
 		case Opt_bsd_df:
@@ -1518,10 +1523,11 @@ set_qf_format:
 			clear_opt(sbi->s_mount_opt, BARRIER);
 			break;
 		case Opt_barrier:
-			if (match_int(&args[0], &option)) {
-				set_opt(sbi->s_mount_opt, BARRIER);
-				break;
-			}
+			if (args[0].from) {
+				if (match_int(&args[0], &option))
+					return 0;
+			} else
+				option = 1;	/* No argument, default to 1 */
 			if (option)
 				set_opt(sbi->s_mount_opt, BARRIER);
 			else
@@ -1594,10 +1600,11 @@ set_qf_format:
 			set_opt(sbi->s_mount_opt,NO_AUTO_DA_ALLOC);
 			break;
 		case Opt_auto_da_alloc:
-			if (match_int(&args[0], &option)) {
-				clear_opt(sbi->s_mount_opt, NO_AUTO_DA_ALLOC);
-				break;
-			}
+			if (args[0].from) {
+				if (match_int(&args[0], &option))
+					return 0;
+			} else
+				option = 1;	/* No argument, default to 1 */
 			if (option)
 				clear_opt(sbi->s_mount_opt, NO_AUTO_DA_ALLOC);
 			else