[PATCH] xfs_copy: don't use cached buffer reads until after libxfs_mount

[PATCH] xfs_copy: don't use cached buffer reads until after libxfs_mount

[Darrick J. Wong]

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) {

[PATCH] xfs: test xfs_copy doesn't do cached read before libxfs_mount

[Darrick J. Wong]

From: Darrick J. Wong <djwong@kernel.org>

This is a regression test for an xfs_copy fix that ensures that it
doesn't perform a cached read of an XFS filesystem prior to initializing
libxfs, since the xfs_mount (and hence the buffer cache) isn't set up
yet.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 tests/xfs/844     |   37 +++++++++++++++++++++++++++++++++++++
 tests/xfs/844.out |    3 +++
 3 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100755 tests/xfs/844
 create mode 100644 tests/xfs/844.out

diff --git a/tests/xfs/844 b/tests/xfs/844
new file mode 100755
index 00000000..720f45bb
--- /dev/null
+++ b/tests/xfs/844
@@ -0,0 +1,37 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2022 Oracle.  All Rights Reserved.
+#
+# FS QA Test 844
+#
+# Regression test for xfsprogs commit:
+#
+# XXXXXXXX ("xfs_copy: don't use cached buffer reads until after libxfs_mount")
+#
+. ./common/preamble
+_begin_fstest auto copy
+
+_cleanup()
+{
+	cd /
+	rm -r -f $tmp.* $TEST_DIR/$seq.*
+}
+
+# Import common functions.
+# . ./common/filter
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs generic
+_require_xfs_copy
+_require_test
+
+truncate -s 100m $TEST_DIR/$seq.a
+truncate -s 100m $TEST_DIR/$seq.b
+
+$XFS_COPY_PROG $TEST_DIR/$seq.a $TEST_DIR/$seq.b
+
+# success, all done
+status=0
+exit
diff --git a/tests/xfs/844.out b/tests/xfs/844.out
new file mode 100644
index 00000000..dbefde1c
--- /dev/null
+++ b/tests/xfs/844.out
@@ -0,0 +1,3 @@
+QA output created by 844
+bad magic number
+xfs_copy: couldn't read superblock, error=22

Re: [PATCH] xfs: test xfs_copy doesn't do cached read before libxfs_mount

[Dave Chinner]

On Tue, May 24, 2022 at 12:54:17PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> This is a regression test for an xfs_copy fix that ensures that it
> doesn't perform a cached read of an XFS filesystem prior to initializing
> libxfs, since the xfs_mount (and hence the buffer cache) isn't set up
> yet.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  tests/xfs/844     |   37 +++++++++++++++++++++++++++++++++++++
>  tests/xfs/844.out |    3 +++
>  3 files changed, 40 insertions(+), 1 deletion(-)
>  create mode 100755 tests/xfs/844
>  create mode 100644 tests/xfs/844.out
> 
> diff --git a/tests/xfs/844 b/tests/xfs/844
> new file mode 100755
> index 00000000..720f45bb
> --- /dev/null
> +++ b/tests/xfs/844
> @@ -0,0 +1,37 @@
> +#! /bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (c) 2022 Oracle.  All Rights Reserved.
> +#
> +# FS QA Test 844
> +#
> +# Regression test for xfsprogs commit:
> +#
> +# XXXXXXXX ("xfs_copy: don't use cached buffer reads until after libxfs_mount")
> +#
> +. ./common/preamble
> +_begin_fstest auto copy
> +
> +_cleanup()
> +{
> +	cd /
> +	rm -r -f $tmp.* $TEST_DIR/$seq.*
> +}

Not necessary - $TEST_DIR/$seq.* are sparse files that take up no space.

> +
> +# Import common functions.
> +# . ./common/filter
> +
> +# real QA test starts here
> +
> +# Modify as appropriate.
> +_supported_fs generic
> +_require_xfs_copy
> +_require_test
> +

rm -f $TEST_DIR/$seq.*

To set up known initial state prior to starting the test.

Otherwise looks OK.

Cheers,

Dave.

> +truncate -s 100m $TEST_DIR/$seq.a
> +truncate -s 100m $TEST_DIR/$seq.b
> +
> +$XFS_COPY_PROG $TEST_DIR/$seq.a $TEST_DIR/$seq.b
> +
> +# success, all done
> +status=0
> +exit
> diff --git a/tests/xfs/844.out b/tests/xfs/844.out
> new file mode 100644
> index 00000000..dbefde1c
> --- /dev/null
> +++ b/tests/xfs/844.out
> @@ -0,0 +1,3 @@
> +QA output created by 844
> +bad magic number
> +xfs_copy: couldn't read superblock, error=22
> 

-- 
Dave Chinner
david@fromorbit.com

Re: [PATCH] xfs: test xfs_copy doesn't do cached read before libxfs_mount

[Zorro Lang]

On Tue, May 24, 2022 at 12:54:17PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> This is a regression test for an xfs_copy fix that ensures that it
> doesn't perform a cached read of an XFS filesystem prior to initializing
> libxfs, since the xfs_mount (and hence the buffer cache) isn't set up
> yet.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  tests/xfs/844     |   37 +++++++++++++++++++++++++++++++++++++
>  tests/xfs/844.out |    3 +++
>  3 files changed, 40 insertions(+), 1 deletion(-)
>  create mode 100755 tests/xfs/844
>  create mode 100644 tests/xfs/844.out
> 
> diff --git a/tests/xfs/844 b/tests/xfs/844
> new file mode 100755
> index 00000000..720f45bb
> --- /dev/null
> +++ b/tests/xfs/844
> @@ -0,0 +1,37 @@
> +#! /bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (c) 2022 Oracle.  All Rights Reserved.
> +#
> +# FS QA Test 844
> +#
> +# Regression test for xfsprogs commit:
> +#
> +# XXXXXXXX ("xfs_copy: don't use cached buffer reads until after libxfs_mount")

     ^^^ I'd like to merge a test after it's fixed offically at first, anyway that
     doesn't prevent us from reviewing it at first :)

> +#
> +. ./common/preamble
> +_begin_fstest auto copy
> +
> +_cleanup()
> +{
> +	cd /
> +	rm -r -f $tmp.* $TEST_DIR/$seq.*
> +}
> +
> +# Import common functions.
> +# . ./common/filter
> +
> +# real QA test starts here
> +
> +# Modify as appropriate.

"Modify as appropriate." can be removed.

> +_supported_fs generic
> +_require_xfs_copy
> +_require_test
> +
> +truncate -s 100m $TEST_DIR/$seq.a
> +truncate -s 100m $TEST_DIR/$seq.b

fstests recommends using "truncate" of XFS_IO_PROG, although use 'truncate'
isn't wrong at here.

As TEST_DIR might have existed files, do we need to do
        rm -rf $TEST_DIR/$seq.a $TEST_DIR/$seq.b
at first?

Thanks,
Zorro

> +
> +$XFS_COPY_PROG $TEST_DIR/$seq.a $TEST_DIR/$seq.b
> +
> +# success, all done
> +status=0
> +exit
> diff --git a/tests/xfs/844.out b/tests/xfs/844.out
> new file mode 100644
> index 00000000..dbefde1c
> --- /dev/null
> +++ b/tests/xfs/844.out
> @@ -0,0 +1,3 @@
> +QA output created by 844
> +bad magic number
> +xfs_copy: couldn't read superblock, error=22
> 

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

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>