btrfs-show-super: don't try to print not-superblocks

index d4df0ac..023f099 100644
@@ -40,16 +40,17 @@
 static void print_usage(void);
 static void dump_superblock(struct btrfs_super_block *sb);
 int main(int argc, char **argv);
-static int load_and_dump_sb(char *, int fd, u64 sb_bytenr);
+static int load_and_dump_sb(char *, int fd, u64 sb_bytenr, int force);
 
 
 static void print_usage(void)
 {
 	fprintf(stderr,
-		"usage: btrfs-show-super [-i super_mirror|-a] dev [dev..]\n");
+		"usage: btrfs-show-super [-i super_mirror|-a] [-f] dev [dev..]\n");
 	fprintf(stderr, "\tThe super_mirror number is between 0 and %d.\n",
 		BTRFS_SUPER_MIRROR_MAX - 1);
-	fprintf(stderr, "\tIf -a is passed all the superblocks are showed.\n");
+	fprintf(stderr, "\tIf -a is passed all the superblocks are shown.\n");
+	fprintf(stderr, "\tPass -f to attempt to dump superblocks with bad magic.\n");
 	fprintf(stderr, "%s\n", BTRFS_BUILD_VERSION);
 }
 

@@ -60,10 +61,11 @@ int main(int argc, char **argv)
 	char *filename;
 	int fd = -1;
 	int i;
+	int force = 0;
 	u64 arg;
 	u64 sb_bytenr = btrfs_sb_offset(0);
 
-	while ((opt = getopt(argc, argv, "ai:")) != -1) {
+	while ((opt = getopt(argc, argv, "afi:")) != -1) {
 		switch (opt) {
 		case 'i':
 			arg = arg_strtou64(optarg);

@@ -81,6 +83,10 @@ int main(int argc, char **argv)
 			all = 1;
 			break;
 
+		case 'f':
+			force = 1;
+			break;
+
 		default:
 			print_usage();
 			exit(1);

@@ -104,7 +110,8 @@ int main(int argc, char **argv)
 			int idx;
 			for (idx = 0; idx < BTRFS_SUPER_MIRROR_MAX; idx++) {
 				sb_bytenr = btrfs_sb_offset(idx);
-				if (load_and_dump_sb(filename, fd, sb_bytenr)) {
+				if (load_and_dump_sb(filename, fd,
+						     sb_bytenr, force)) {
 					close(fd);
 					exit(1);
 				}

@@ -112,7 +119,7 @@ int main(int argc, char **argv)
 				putchar('\n');
 			}
 		} else {
-			load_and_dump_sb(filename, fd, sb_bytenr);
+			load_and_dump_sb(filename, fd, sb_bytenr, force);
 			putchar('\n');
 		}
 		close(fd);

@@ -121,7 +128,7 @@ int main(int argc, char **argv)
 	exit(0);
 }
 
-static int load_and_dump_sb(char *filename, int fd, u64 sb_bytenr)
+static int load_and_dump_sb(char *filename, int fd, u64 sb_bytenr, int force)
 {
 	u8 super_block_data[BTRFS_SUPER_INFO_SIZE];
 	struct btrfs_super_block *sb;

@@ -146,7 +153,12 @@ static int load_and_dump_sb(char *filename, int fd, u64 sb_bytenr)
 	}
 	printf("superblock: bytenr=%llu, device=%s\n", sb_bytenr, filename);
 	printf("---------------------------------------------------------\n");
-	dump_superblock(sb);
+	if (btrfs_super_magic(sb) != BTRFS_MAGIC && !force) {
+		fprintf(stderr,
+		    "ERROR: bad magic on superblock on %s at %llu\n",
+		    filename, (unsigned long long)sb_bytenr);
+	} else
+		dump_superblock(sb);
 	return 0;
 }