From: Darrick J. Wong <djwong@xxxxxxxxxx>
I accidentally tried to xfs_copy an ext4 filesystem, but instead of
rejecting the filesystem, the program instead crashed. I figured out
that zeroing the superblock was enough to trigger this:
# dd if=/dev/zero of=/dev/sda bs=1024k count=1
# xfs_copy /dev/sda /dev/sdb
Floating point exception
The exact crash happens in this line from libxfs_getbuf_flags, which is
called from the main() routine of xfs_copy:
if (btp == btp->bt_mount->m_ddev_targp) {
(*bpp)->b_pag = xfs_perag_get(btp->bt_mount,
xfs_daddr_to_agno(btp->bt_mount, blkno));
The problem here is that the uncached read filled the incore superblock
with zeroes, which means mbuf.sb_agblocks is zero. This causes a
division by zero in xfs_daddr_to_agno, thereby crashing the program.
In commit f8b581d6, we made it so that xfs_buf structures contain a
passive reference to the associated perag structure. That commit
assumes that no program would try a cached buffer read until the buffer
cache is fully set up, which is true throughout xfsprogs... except for
the beginning of xfs_copy. For whatever reason, it attempts an uncached
read of the superblock to figure out the real superblock size, then
performs a *cached* read with the proper buffer length and verifier.
The cached read crashes the program.
Fix the problem by changing the (second) cached read into an uncached read.
Fixes: f8b581d6 ("libxfs: actually make buffers track the per-ag structures")
Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
---
copy/xfs_copy.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/copy/xfs_copy.c b/copy/xfs_copy.c
index 41f594bd..79f65946 100644
--- a/copy/xfs_copy.c
+++ b/copy/xfs_copy.c
@@ -748,7 +748,7 @@ main(int argc, char **argv)
/* Do it again, now with proper length and verifier */
libxfs_buf_relse(sbp);
- error = -libxfs_buf_read(mbuf.m_ddev_targp, XFS_SB_DADDR,
+ error = -libxfs_buf_read_uncached(mbuf.m_ddev_targp, XFS_SB_DADDR,
1 << (sb->sb_sectlog - BBSHIFT), 0, &sbp,
&xfs_sb_buf_ops);
if (error) {
