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authorBrian Foster <bfoster@redhat.com>2017-04-20 08:06:47 -0700
committerDarrick J. Wong <darrick.wong@oracle.com>2017-04-25 09:40:40 -0700
commitcb52ee334a45ae6c78a3999e4b473c43ddc528f4 (patch)
treedfd3ef297c8bdb4b9443a4a57945119966527010
parent023cc840b40fad95c6fe26fff1d380a8c9d45939 (diff)
xfs: prevent multi-fsb dir readahead from reading random blocks
Directory block readahead uses a complex iteration mechanism to map between high-level directory blocks and underlying physical extents. This mechanism attempts to traverse the higher-level dir blocks in a manner that handles multi-fsb directory blocks and simultaneously maintains a reference to the corresponding physical blocks. This logic doesn't handle certain (discontiguous) physical extent layouts correctly with multi-fsb directory blocks. For example, consider the case of a 4k FSB filesystem with a 2 FSB (8k) directory block size and a directory with the following extent layout: EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL 0: [0..7]: 88..95 0 (88..95) 8 1: [8..15]: 80..87 0 (80..87) 8 2: [16..39]: 168..191 0 (168..191) 24 3: [40..63]: 5242952..5242975 1 (72..95) 24 Directory block 0 spans physical extents 0 and 1, dirblk 1 lies entirely within extent 2 and dirblk 2 spans extents 2 and 3. Because extent 2 is larger than the directory block size, the readahead code erroneously assumes the block is contiguous and issues a readahead based on the physical mapping of the first fsb of the dirblk. This results in read verifier failure and a spurious corruption or crc failure, depending on the filesystem format. Further, the subsequent readahead code responsible for walking through the physical table doesn't correctly advance the physical block reference for dirblk 2. Instead of advancing two physical filesystem blocks, the first iteration of the loop advances 1 block (correctly), but the subsequent iteration advances 2 more physical blocks because the next physical extent (extent 3, above) happens to cover more than dirblk 2. At this point, the higher-level directory block walking is completely off the rails of the actual physical layout of the directory for the respective mapping table. Update the contiguous dirblock logic to consider the current offset in the physical extent to avoid issuing directory readahead to unrelated blocks. Also, update the mapping table advancing code to consider the current offset within the current dirblock to avoid advancing the mapping reference too far beyond the dirblock. Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
-rw-r--r--fs/xfs/xfs_dir2_readdir.c5
1 files changed, 3 insertions, 2 deletions
diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
index c45de729c74e..20b7a5c6eb2f 100644
--- a/fs/xfs/xfs_dir2_readdir.c
+++ b/fs/xfs/xfs_dir2_readdir.c
@@ -405,7 +405,8 @@ xfs_dir2_leaf_readbuf(
* Read-ahead a contiguous directory block.
*/
if (i > mip->ra_current &&
- map[mip->ra_index].br_blockcount >= geo->fsbcount) {
+ (map[mip->ra_index].br_blockcount - mip->ra_offset) >=
+ geo->fsbcount) {
xfs_dir3_data_readahead(dp,
map[mip->ra_index].br_startoff + mip->ra_offset,
XFS_FSB_TO_DADDR(dp->i_mount,
@@ -438,7 +439,7 @@ xfs_dir2_leaf_readbuf(
* The rest of this extent but not more than a dir
* block.
*/
- length = min_t(int, geo->fsbcount,
+ length = min_t(int, geo->fsbcount - j,
map[mip->ra_index].br_blockcount -
mip->ra_offset);
mip->ra_offset += length;