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

If we point btrfs-show-super at a not-btrfs-device and
try to print all superblocks, bad things are apt to happen:

superblock: bytenr=274877906944, device=/dev/sdc2
---------------------------------------------------------
btrfs-show-super: ctree.h:1984: btrfs_super_csum_size: \
Assertion `!(t >= (sizeof(btrfs_csum_sizes) / sizeof((btrfs_csum_sizes)[0])))' failed.
csum			0xAborted

Don't try to print superblocks that don't look like superblocks,
and add an "-f" (force) option to try anyway, if the user
really wants to give it a shot.

Fix some spelling & capitalization while we're at it.

The manpage says that if any problem happens, 1 will
be returned, but that's already not true today LOL, so
I didn't bother to make it true when we detect bad
sb magic, either...

I figure it's worth continuing and trying all superblocks
in case just one has a corrupt magic.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[renamed -f to -F due to clash with existing option, converted
relevant docs to asciidoc]
Signed-off-by: David Sterba <dsterba@suse.cz>

Documentation/btrfs-show-super.txt

@@ -31,6 +31,10 @@ Specify which mirror to print out.
 <super_mirror> is between 0 and 2.
 If several '-i <super_mirror>' are given, only the last one is valid.
 
+-F::
+Attempt to print the superblock even if no superblock magic is found.  May end
+badly.
+
 EXIT STATUS
 -----------
 *btrfs-show-super* will return 0 if no error happened.

btrfs-show-super.c

@@ -40,16 +40,17 @@
 static void print_usage(void);
 static void dump_superblock(struct btrfs_super_block *sb, int full);
 int main(int argc, char **argv);
-static int load_and_dump_sb(char *, int fd, u64 sb_bytenr, int full);
+static int load_and_dump_sb(char *, int fd, u64 sb_bytenr, int full, int force);
 
 
 static void print_usage(void)
 {
 	fprintf(stderr,
-		"usage: btrfs-show-super [-i super_mirror|-a|-f] dev [dev..]\n");
+		"usage: btrfs-show-super [-i super_mirror|-a|-f|-F] dev [dev..]\n");
 	fprintf(stderr, "\t-f : print full superblock information\n");
 	fprintf(stderr, "\t-a : print information of all superblocks\n");
 	fprintf(stderr, "\t-i <super_mirror> : specify which mirror to print out\n");
+	fprintf(stderr, "\t-F : attempt to dump superblocks with bad magic\n");
 	fprintf(stderr, "%s\n", BTRFS_BUILD_VERSION);
 }
 
@@ -58,13 +59,14 @@ int main(int argc, char **argv)
 	int opt;
 	int all = 0;
 	int full = 0;
+	int force = 0;
 	char *filename;
 	int fd = -1;
 	int i;
 	u64 arg;
 	u64 sb_bytenr = btrfs_sb_offset(0);
 
-	while ((opt = getopt(argc, argv, "fai:")) != -1) {
+	while ((opt = getopt(argc, argv, "fFai:")) != -1) {
 		switch (opt) {
 		case 'i':
 			arg = arg_strtou64(optarg);
@@ -84,6 +86,9 @@ int main(int argc, char **argv)
 		case 'f':
 			full = 1;
 			break;
+		case 'F':
+			force = 1;
+			break;
 		default:
 			print_usage();
 			exit(1);
@@ -108,7 +113,7 @@ int main(int argc, char **argv)
 			for (idx = 0; idx < BTRFS_SUPER_MIRROR_MAX; idx++) {
 				sb_bytenr = btrfs_sb_offset(idx);
 				if (load_and_dump_sb(filename, fd,
-							sb_bytenr, full)) {
+						sb_bytenr, full, force)) {
 					close(fd);
 					exit(1);
 				}
@@ -116,7 +121,7 @@ int main(int argc, char **argv)
 				putchar('\n');
 			}
 		} else {
-			load_and_dump_sb(filename, fd, sb_bytenr, full);
+			load_and_dump_sb(filename, fd, sb_bytenr, full, force);
 			putchar('\n');
 		}
 		close(fd);
@@ -125,7 +130,8 @@ int main(int argc, char **argv)
 	exit(0);
 }
 
-static int load_and_dump_sb(char *filename, int fd, u64 sb_bytenr, int full)
+static int load_and_dump_sb(char *filename, int fd, u64 sb_bytenr, int full,
+		int force)
 {
 	u8 super_block_data[BTRFS_SUPER_INFO_SIZE];
 	struct btrfs_super_block *sb;
@@ -150,7 +156,13 @@ static int load_and_dump_sb(char *filename, int fd, u64 sb_bytenr, int full)
 	}
 	printf("superblock: bytenr=%llu, device=%s\n", sb_bytenr, filename);
 	printf("---------------------------------------------------------\n");
+	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, full);
+	}
 	return 0;
 }