[PATCH RFC] xfs: fix buffer check for primary sb in userspace libxfs

Brian Foster
2017-07-18T14:13:38Z

Signed-off-by: Brian Foster <bfoster@redhat.com>
---

Hi all,

This patch is actually targeted at userspace. The previous change in commit
f3d7ebde ("xfs: fix superblock inprogress check") to use ->b_maps technically
breaks the logic in userspace in a similar way to the original problem because
userspace has no concept of uncached buffers. ->b_maps is NULL in userspace
unless the buffer is truly discontiguous.

This would normally result in a segfault but this appears to be hidden
by gcc optimization as -O2 is enabled by default and the
check_inprogress param to xfs_mount_validate_sb() is unused in
userspace. Therefore, the segfault is only reproducible when
optimization is disabled (which is a useful configuration for
debugging).

There are obviously different ways to fix this. I'm floating this (untested)
rfc as a kernel patch (do we ever sync libxfs from xfsprogs -> kernel?) with
the objective of keeping the libxfs code the same between the kernel and
userspace. We could alternatively create a custom helper/macro with the
appropriate check in each place. Thoughts?

Brian

fs/xfs/libxfs/xfs_sb.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
index 9b5aae2..ec2fd03 100644
--- a/fs/xfs/libxfs/xfs_sb.c
+++ b/fs/xfs/libxfs/xfs_sb.c
@@ -583,6 +583,7 @@ xfs_sb_verify(
{
struct xfs_mount *mp = bp->b_target->bt_mount;
struct xfs_sb sb;
+ bool primary_sb;

/*
* Use call variant which doesn't convert quota flags from disk
@@ -592,11 +593,14 @@ xfs_sb_verify(

/*
* Only check the in progress field for the primary superblock as
- * mkfs.xfs doesn't clear it from secondary superblocks.
+ * mkfs.xfs doesn't clear it from secondary superblocks. Note that
+ * userspace libxfs does not have uncached buffers and so b_maps is not
+ * used for the sb buffer.
*/
- return xfs_mount_validate_sb(mp, &sb,
- bp->b_maps[0].bm_bn == XFS_SB_DADDR,
- check_version);
+ primary_sb = (bp->b_bn == XFS_BUF_DADDR_NULL &&
+ bp->b_maps[0].bm_bn == XFS_SB_DADDR) ||
+ bp->b_bn == XFS_SB_DADDR;
+ return xfs_mount_validate_sb(mp, &sb, primary_sb, check_version);
}

/*
--
2.9.4