xfs: reset buffer pointers before freeing them

When we free a vmapped buffer, we need to ensure the vmap address
and length we free is the same as when it was allocated. In various
places in the log code we change the memory the buffer is pointing
to before issuing IO, but we never reset the buffer to point back to
it's original memory (or no memory, if that is the case for the
buffer).

As a result, when we free the buffer it points to memory that is
owned by something else and attempts to unmap and free it. Because
the range does not match any known mapped range, it can trigger
BUG_ON() traps in the vmap code, and potentially corrupt the vmap
area tracking.

Fix this by always resetting these buffers to their original state
before freeing them.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/linux-2.6/xfs_buf.c

@@ -708,6 +708,27 @@ xfs_buf_get_empty(
 	return bp;
 }
 
+/*
+ * Return a buffer allocated as an empty buffer and associated to external
+ * memory via xfs_buf_associate_memory() back to it's empty state.
+ */
+void
+xfs_buf_set_empty(
+	struct xfs_buf		*bp,
+	size_t			len)
+{
+	if (bp->b_pages)
+		_xfs_buf_free_pages(bp);
+
+	bp->b_pages = NULL;
+	bp->b_page_count = 0;
+	bp->b_addr = NULL;
+	bp->b_file_offset = 0;
+	bp->b_buffer_length = bp->b_count_desired = len;
+	bp->b_bn = XFS_BUF_DADDR_NULL;
+	bp->b_flags &= ~XBF_MAPPED;
+}
+
 static inline struct page *
 mem_to_page(
 	void			*addr)

fs/xfs/linux-2.6/xfs_buf.h

@@ -178,6 +178,7 @@ extern xfs_buf_t *xfs_buf_read(xfs_buftarg_t *, xfs_off_t, size_t,
 				xfs_buf_flags_t);
 
 extern xfs_buf_t *xfs_buf_get_empty(size_t, xfs_buftarg_t *);
+extern void xfs_buf_set_empty(struct xfs_buf *bp, size_t len);
 extern xfs_buf_t *xfs_buf_get_uncached(struct xfs_buftarg *, size_t, int);
 extern int xfs_buf_associate_memory(xfs_buf_t *, void *, size_t);
 extern void xfs_buf_hold(xfs_buf_t *);

fs/xfs/xfs_log.c

@@ -1449,6 +1449,13 @@ xlog_dealloc_log(xlog_t *log)
 
 	xlog_cil_destroy(log);
 
+	/*
+	 * always need to ensure that the extra buffer does not point to memory
+	 * owned by another log buffer before we free it.
+	 */
+	xfs_buf_set_empty(log->l_xbuf, log->l_iclog_size);
+	xfs_buf_free(log->l_xbuf);
+
 	iclog = log->l_iclog;
 	for (i=0; i<log->l_iclog_bufs; i++) {
 		xfs_buf_free(iclog->ic_bp);
@@ -1458,7 +1465,6 @@ xlog_dealloc_log(xlog_t *log)
 	}
 	spinlock_destroy(&log->l_icloglock);
 
-	xfs_buf_free(log->l_xbuf);
 	log->l_mp->m_log = NULL;
 	kmem_free(log);
 }	/* xlog_dealloc_log */

fs/xfs/xfs_log_recover.c

@@ -204,6 +204,35 @@ xlog_bread(
 	return 0;
 }
 
+/*
+ * Read at an offset into the buffer. Returns with the buffer in it's original
+ * state regardless of the result of the read.
+ */
+STATIC int
+xlog_bread_offset(
+	xlog_t		*log,
+	xfs_daddr_t	blk_no,		/* block to read from */
+	int		nbblks,		/* blocks to read */
+	xfs_buf_t	*bp,
+	xfs_caddr_t	offset)
+{
+	xfs_caddr_t	orig_offset = XFS_BUF_PTR(bp);
+	int		orig_len = bp->b_buffer_length;
+	int		error, error2;
+
+	error = XFS_BUF_SET_PTR(bp, offset, BBTOB(nbblks));
+	if (error)
+		return error;
+
+	error = xlog_bread_noalign(log, blk_no, nbblks, bp);
+
+	/* must reset buffer pointer even on error */
+	error2 = XFS_BUF_SET_PTR(bp, orig_offset, orig_len);
+	if (error)
+		return error;
+	return error2;
+}
+
 /*
  * Write out the buffer at the given block for the given number of blocks.
  * The buffer is kept locked across the write and is returned locked.
@@ -1229,20 +1258,12 @@ xlog_write_log_records(
 		 */
 		ealign = round_down(end_block, sectbb);
 		if (j == 0 && (start_block + endcount > ealign)) {
-			offset = XFS_BUF_PTR(bp);
-			balign = BBTOB(ealign - start_block);
-			error = XFS_BUF_SET_PTR(bp, offset + balign,
-						BBTOB(sectbb));
+			offset = XFS_BUF_PTR(bp) + BBTOB(ealign - start_block);
+			error = xlog_bread_offset(log, ealign, sectbb,
+							bp, offset);
 			if (error)
 				break;
 
-			error = xlog_bread_noalign(log, ealign, sectbb, bp);
-			if (error)
-				break;
-
-			error = XFS_BUF_SET_PTR(bp, offset, bufblks);
-			if (error)
-				break;
 		}
 
 		offset = xlog_align(log, start_block, endcount, bp);
@@ -3448,19 +3469,9 @@ xlog_do_recovery_pass(
 				 *   - order is important.
 				 */
 				wrapped_hblks = hblks - split_hblks;
-				error = XFS_BUF_SET_PTR(hbp,
-						offset + BBTOB(split_hblks),
-						BBTOB(hblks - split_hblks));
-				if (error)
-					goto bread_err2;
-
-				error = xlog_bread_noalign(log, 0,
-							   wrapped_hblks, hbp);
-				if (error)
-					goto bread_err2;
-
-				error = XFS_BUF_SET_PTR(hbp, offset,
-							BBTOB(hblks));
+				error = xlog_bread_offset(log, 0,
+						wrapped_hblks, hbp,
+						offset + BBTOB(split_hblks));
 				if (error)
 					goto bread_err2;
 			}
@@ -3511,19 +3522,9 @@ xlog_do_recovery_pass(
 				 *   _first_, then the log start (LR header end)
 				 *   - order is important.
 				 */
-				error = XFS_BUF_SET_PTR(dbp,
-						offset + BBTOB(split_bblks),
-						BBTOB(bblks - split_bblks));
-				if (error)
-					goto bread_err2;
-
-				error = xlog_bread_noalign(log, wrapped_hblks,
-						bblks - split_bblks,
-						dbp);
-				if (error)
-					goto bread_err2;
-
-				error = XFS_BUF_SET_PTR(dbp, offset, h_size);
+				error = xlog_bread_offset(log, 0,
+						bblks - split_bblks, hbp,
+						offset + BBTOB(split_bblks));
 				if (error)
 					goto bread_err2;
 			}