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Subject[PATCH -mm] nilfs2: make block erasure safe in nilfs_finish_roll_forward()
Date
The implementation of writing a zero-fill block in
nilfs_finish_roll_forward() is not safe. The buffer is being cleared
without acquiring a lock or setting the uptodate flag, so
theoretically, between the time the buffer's data is cleared and the
time it is written back to the block device using sync_dirty_buffer(),
that zero data can be undone by concurrent block device reads.

Since this buffer points to a location that has been read from disk
once, the uptodate flag will most likely remain, but since it was
obtained with __getblk(), that is not guaranteed. In other words,
this is exceptional, and this function itself is not normally called
(only once when mounting after a specific pattern of unclean
shutdown), so it is highly unlikely that this will actually cause a
problem.

Anyway, eliminate this potential race issue by protecting the clearing
of buffer data with a buffer lock and setting the buffer's uptodate
flag within the protected section.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
Andrew, please add another patch to your queue for the next cycle
(or as a bug fix if it's late). It eliminates one potential race issue.

Thanks,
Ryusuke Konishi

fs/nilfs2/recovery.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/fs/nilfs2/recovery.c b/fs/nilfs2/recovery.c
index 020f304c600e..b638dc06df2f 100644
--- a/fs/nilfs2/recovery.c
+++ b/fs/nilfs2/recovery.c
@@ -702,8 +702,12 @@ static void nilfs_finish_roll_forward(struct the_nilfs *nilfs,
if (WARN_ON(!bh))
return; /* should never happen */

+ lock_buffer(bh);
memset(bh->b_data, 0, bh->b_size);
+ set_buffer_uptodate(bh);
set_buffer_dirty(bh);
+ unlock_buffer(bh);
+
err = sync_dirty_buffer(bh);
if (unlikely(err))
nilfs_warn(nilfs->ns_sb,
--
2.34.1

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