From: Vladimir Sementsov-Ogievskiy Date: Thu, 6 Feb 2020 19:42:45 +0300 Subject: block: fix crash on zero-length unaligned write and read Git-commit: ac9d00bf7b47acae6b0e42910d9ed55fef3af5b8 Commit 7a3f542fbd "block/io: refactor padding" occasionally dropped aligning for zero-length request: bdrv_init_padding() blindly return false if bytes == 0, like there is nothing to align. This leads the following command to crash: ./qemu-io --image-opts -c 'write 1 0' \ driver=blkdebug,align=512,image.driver=null-co,image.size=512 >> qemu-io: block/io.c:1955: bdrv_aligned_pwritev: Assertion `(offset & (align - 1)) == 0' failed. >> Aborted (core dumped) Prior to 7a3f542fbd we does aligning of such zero requests. Instead of recovering this behavior let's just do nothing on such requests as it is useless. Note that driver may have special meaning of zero-length reqeusts, like qcow2_co_pwritev_compressed_part, so we can't skip any zero-length operation. But for unaligned ones, we can't pass it to driver anyway. This commit also fixes crash in iotest 80 running with -nocache: ./check -nocache -qcow2 80 which crashes on same assertion due to trying to read empty extra data in qcow2_do_read_snapshots(). Cc: qemu-stable@nongnu.org # v4.2 Fixes: 7a3f542fbd Signed-off-by: Vladimir Sementsov-Ogievskiy Reviewed-by: Max Reitz Message-id: 20200206164245.17781-1-vsementsov@virtuozzo.com Signed-off-by: Stefan Hajnoczi Signed-off-by: Bruce Rogers --- block/io.c | 28 +++++++++++++++++++++++++++- 1 file changed, 27 insertions(+), 1 deletion(-) diff --git a/block/io.c b/block/io.c index f75777f5ea744aac5c9b5872f203..d1f1ee9138c7af82352e9277f22b 100644 --- a/block/io.c +++ b/block/io.c @@ -1567,10 +1567,12 @@ static bool bdrv_init_padding(BlockDriverState *bs, pad->tail = align - pad->tail; } - if ((!pad->head && !pad->tail) || !bytes) { + if (!pad->head && !pad->tail) { return false; } + assert(bytes); /* Nothing good in aligning zero-length requests */ + sum = pad->head + bytes + pad->tail; pad->buf_len = (sum > align && pad->head && pad->tail) ? 2 * align : align; pad->buf = qemu_blockalign(bs, pad->buf_len); @@ -1708,6 +1710,18 @@ int coroutine_fn bdrv_co_preadv_part(BdrvChild *child, return ret; } + if (bytes == 0 && !QEMU_IS_ALIGNED(offset, bs->bl.request_alignment)) { + /* + * Aligning zero request is nonsense. Even if driver has special meaning + * of zero-length (like qcow2_co_pwritev_compressed_part), we can't pass + * it to driver due to request_alignment. + * + * Still, no reason to return an error if someone do unaligned + * zero-length read occasionally. + */ + return 0; + } + bdrv_inc_in_flight(bs); /* Don't do copy-on-read if we read data before write operation */ @@ -2115,6 +2129,18 @@ int coroutine_fn bdrv_co_pwritev_part(BdrvChild *child, return -ENOTSUP; } + if (bytes == 0 && !QEMU_IS_ALIGNED(offset, bs->bl.request_alignment)) { + /* + * Aligning zero request is nonsense. Even if driver has special meaning + * of zero-length (like qcow2_co_pwritev_compressed_part), we can't pass + * it to driver due to request_alignment. + * + * Still, no reason to return an error if someone do unaligned + * zero-length write occasionally. + */ + return 0; + } + bdrv_inc_in_flight(bs); /* * Align write if necessary by performing a read-modify-write cycle.