block: fix crash on zero-length unaligned write and read
Commit 7a3f542f "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 7a3f542f 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: 7a3f542fSigned-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> Reviewed-by: NMax Reitz <mreitz@redhat.com> Message-id: 20200206164245.17781-1-vsementsov@virtuozzo.com Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com> (cherry picked from commit ac9d00bf) Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
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