[PATCH] xfs: ensure correct log item buffer alignment

[PATCH] xfs: ensure correct log item buffer alignment

[Dave Chinner]

From: Dave Chinner <dchinner@redhat.com>

On 32 bit platforms, the log item vector headers are not 64 bit
aligned or sized. hence if we don't take care to align them
correctly or pad the buffer appropriately for 8 byte alignment, we
can end up with alignment issues when accessing the user buffer
directly as a structure.

To solve this, simply pad the buffer headers to 64 bit offset so
that the data section is always 8 byte aligned.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/xfs_log_cil.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index cdebd83..4ef6fdb 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -205,16 +205,25 @@ xlog_cil_insert_format_items(
 		/*
 		 * We 64-bit align the length of each iovec so that the start
 		 * of the next one is naturally aligned.  We'll need to
-		 * account for that slack space here.
+		 * account for that slack space here. Then round nbytes up
+		 * to 64-bit alignment so that the initial buffer alignment is
+		 * easy to calculate and verify.
 		 */
 		nbytes += niovecs * sizeof(uint64_t);
+		nbytes = round_up(nbytes, sizeof(uint64_t));
 
 		/* grab the old item if it exists for reservation accounting */
 		old_lv = lip->li_lv;
 
-		/* calc buffer size */
-		buf_size = sizeof(struct xfs_log_vec) + nbytes +
-				niovecs * sizeof(struct xfs_log_iovec);
+		/*
+		 * The data buffer needs to start 64-bit aligned, so round up
+		 * that space to ensure we can align it appropriately and not
+		 * overrun the buffer.
+		 */
+		buf_size = nbytes +
+			   round_up((sizeof(struct xfs_log_vec) +
+				     niovecs * sizeof(struct xfs_log_iovec)),
+				    sizeof(uint64_t));
 
 		/* compare to existing item size */
 		if (lip->li_lv && buf_size <= lip->li_lv->lv_size) {
@@ -251,6 +260,8 @@ xlog_cil_insert_format_items(
 		/* The allocated data region lies beyond the iovec region */
 		lv->lv_buf_len = 0;
 		lv->lv_buf = (char *)lv + buf_size - nbytes;
+		ASSERT(IS_ALIGNED((unsigned long)lv->lv_buf, sizeof(uint64_t)));
+
 		lip->li_ops->iop_format(lip, lv);
 insert:
 		ASSERT(lv->lv_buf_len <= nbytes);
-- 
1.8.4.rc3

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Re: [PATCH] xfs: ensure correct log item buffer alignment

[Christoph Hellwig]

Looks good,

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

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