Re: [PATCH 1/6] xfs_copy: don't use cached buffer reads until after libxfs_mount

[Dave Chinner <david@fromorbit.com>]

On Tue, Jun 28, 2022 at 01:49:20PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> 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@kernel.org>
> ---
>  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) {

Looks good.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com