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| H A D | dm.c | diff 8dd601fa8317243be887458c49f6c29c2f3d719f Thu Feb 15 03:00:15 CST 2018 NeilBrown <neilb@suse.com> dm: correctly handle chained bios in dec_pending()
dec_pending() is given an error status (possibly 0) to be recorded
against a bio. It can be called several times on the one 'struct
dm_io', and it is careful to only assign a non-zero error to
io->status. However when it then assigned io->status to bio->bi_status,
it is not careful and could overwrite a genuine error status with 0.
This can happen when chained bios are in use. If a bio is chained
beneath the bio that this dm_io is handling, the child bio might
complete and set bio->bi_status before the dm_io completes.
This has been possible since chained bios were introduced in 3.14, and
has become a lot easier to trigger with commit 18a25da84354 ("dm: ensure
bio submission follows a depth-first tree walk") as that commit caused
dm to start using chained bios itself.
A particular failure mode is that if a bio spans an 'error' target and a
working target, the 'error' fragment will complete instantly and set the
->bi_status, and the other fragment will normally complete a little
later, and will clear ->bi_status.
The fix is simply to only assign io_error to bio->bi_status when
io_error is not zero.
Reported-and-tested-by: Milan Broz <gmazyland@gmail.com>
Cc: stable@vger.kernel.org (v3.14+)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
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