xfs: validate metadata LSNs against log on v5 superblocks

Since the onset of v5 superblocks, the LSN of the last modification has
been included in a variety of on-disk data structures. This LSN is used
to provide log recovery ordering guarantees (e.g., to ensure an older
log recovery item is not replayed over a newer target data structure).

While this works correctly from the point a filesystem is formatted and
mounted, userspace tools have some problematic behaviors that defeat
this mechanism. For example, xfs_repair historically zeroes out the log
unconditionally (regardless of whether corruption is detected). If this
occurs, the LSN of the filesystem is reset and the log is now in a
problematic state with respect to on-disk metadata structures that might
have a larger LSN. Until either the log catches up to the highest
previously used metadata LSN or each affected data structure is modified
and written out without incident (which resets the metadata LSN), log
recovery is susceptible to filesystem corruption.

This problem is ultimately addressed and repaired in the associated
userspace tools. The kernel is still responsible to detect the problem
and notify the user that something is wrong. Check the superblock LSN at
mount time and fail the mount if it is invalid. From that point on,
trigger verifier failure on any metadata I/O where an invalid LSN is
detected. This results in a filesystem shutdown and guarantees that we
do not log metadata changes with invalid LSNs on disk. Since this is a
known issue with a known recovery path, present a warning to instruct
the user how to recover.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 950f3f94720c..8daba7491b13 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -560,4 +560,55 @@ static inline void xlog_wait(wait_queue_head_t *wq, spinlock_t *lock)
         remove_wait_queue(wq, &wait);
 }
 
+/*
+ * The LSN is valid so long as it is behind the current LSN. If it isn't, this
+ * means that the next log record that includes this metadata could have a
+ * smaller LSN. In turn, this means that the modification in the log would not
+ * replay.
+ */
+static inline bool
+xlog_valid_lsn(
+        struct xlog     *log,
+        xfs_lsn_t       lsn)
+{
+        int             cur_cycle;
+        int             cur_block;
+        bool            valid = true;
+
+        /*
+         * First, sample the current lsn without locking to avoid added
+         * contention from metadata I/O. The current cycle and block are updated
+         * (in xlog_state_switch_iclogs()) and read here in a particular order
+         * to avoid false negatives (e.g., thinking the metadata LSN is valid
+         * when it is not).
+         *
+         * The current block is always rewound before the cycle is bumped in
+         * xlog_state_switch_iclogs() to ensure the current LSN is never seen in
+         * a transiently forward state. Instead, we can see the LSN in a
+         * transiently behind state if we happen to race with a cycle wrap.
+         */
+        cur_cycle = ACCESS_ONCE(log->l_curr_cycle);
+        smp_rmb();
+        cur_block = ACCESS_ONCE(log->l_curr_block);
+
+        if ((CYCLE_LSN(lsn) > cur_cycle) ||
+            (CYCLE_LSN(lsn) == cur_cycle && BLOCK_LSN(lsn) > cur_block)) {
+                /*
+                 * If the metadata LSN appears invalid, it's possible the check
+                 * above raced with a wrap to the next log cycle. Grab the lock
+                 * to check for sure.
+                 */
+                spin_lock(&log->l_icloglock);
+                cur_cycle = log->l_curr_cycle;
+                cur_block = log->l_curr_block;
+                spin_unlock(&log->l_icloglock);
+
+                if ((CYCLE_LSN(lsn) > cur_cycle) ||
+                    (CYCLE_LSN(lsn) == cur_cycle && BLOCK_LSN(lsn) > cur_block))
+                        valid = false;
+        }
+
+        return valid;
+}
+
 #endif  /* __XFS_LOG_PRIV_H__ */