Re: [PATCH] block: fix crash on zero-length unaligned write and read

[Max Reitz <mreitz@redhat.com>]

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On 06.02.20 17:42, Vladimir Sementsov-Ogievskiy wrote:
> 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 <vsementsov@virtuozzo.com>
> ---
>  block/io.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)

Zero-length reads would still trigger CORs when padded.  But there is no
reason to assume or rely on this, so:

Reviewed-by: Max Reitz <mreitz@redhat.com>

(block/io.c is Stefan’s department. :-))

Max


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