[PATCH, RFC V2] mke2fs: wipe out old btrfs superblocks

[Eric Sandeen <sandeen@redhat.com>]

btrfs sticks superblocks at 64k, 64M, and 256G.  If we don't
overwrite those, libblkid may accidentally identify an ext*
filesystem with old btrfs superblocks as btrfs, and we'll
be sad.

libblkid provides a blkid_wipe_fs() functionality to zero
all existing signatures, but that'd break our handy-dandy
undo capability, I think.  So I'm not sure we have any
other choice but to do it ourselves.

There is a slight error here in that if the mkfs
does not span the entire device, we won't overwrite
signatures past the end of the filesystem, but that case
should be pretty rare.  (The same slight error in logic
applies to the existing "wipe old MD superblock" path).

Signed-off-by: Eric Sandeen <sandeen@redhat.com
Resolves-RHBZ: 902512
---

v2: skip this on "noaction"

I decided not to be tricky about previous zero-returning
discards, there seems to be little point to that, we're
just doing 3 IOs post discard, at worst.

diff --git a/misc/mke2fs.c b/misc/mke2fs.c
index bbf477a..0c67134 100644
--- a/misc/mke2fs.c
+++ b/misc/mke2fs.c
@@ -2307,6 +2307,32 @@ static int mke2fs_discard_device(ext2_filsys fs)
 	return retval;
 }
 
+static int mke2fs_wipe_btrfs(ext2_filsys fs)
+{
+	int blocks;	/* nr of blocks to zero */
+	blk64_t start;	/* location to zero out */
+	int retval = 0; /* accumulate any failures */
+
+	blocks = 1;
+	if (fs->blocksize < 4096)
+		blocks = 4096 / fs->blocksize;
+	/*
+	 * Wipe out any old btrfs superblocks, at
+	 * 64k, 64M, and 256G.
+	 */
+	start = 64ULL * 1024 / fs->blocksize;
+	retval += ext2fs_zero_blocks2(fs, start, blocks, NULL, NULL);
+	start = 64ULL * 1024 * 1024 / fs->blocksize;
+	if (start + blocks <= ext2fs_blocks_count(fs->super))
+		retval += ext2fs_zero_blocks2(fs, start, blocks, NULL, NULL);
+	start = 256ULL * 1024 * 1024 * 1024 / fs->blocksize;
+	if (start + blocks <= ext2fs_blocks_count(fs->super))
+		retval += ext2fs_zero_blocks2(fs, start, blocks, NULL, NULL);
+	/* free the static zeroing buffer */
+	ext2fs_zero_blocks2(0, 0, 0, 0, 0);
+	return retval;
+}
+
 static void fix_cluster_bg_counts(ext2_filsys fs)
 {
 	blk64_t	cluster, num_clusters, tot_free;
@@ -2440,6 +2466,12 @@ int main (int argc, char *argv[])
 		}
 	}
 
+	if (!noaction) {
+		retval = mke2fs_wipe_btrfs(fs);
+		if (retval)
+			printf(_("Failed to wipe old btrfs super locations\n"));
+	}
+
 	sprintf(tdb_string, "tdb_data_size=%d", fs->blocksize <= 4096 ?
 		32768 : fs->blocksize * 8);
 	io_channel_set_options(fs->io, tdb_string);