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Re: [PATCH] block: fix crash on zero-length unaligned write and read
From: |
Max Reitz |
Subject: |
Re: [PATCH] block: fix crash on zero-length unaligned write and read |
Date: |
Thu, 6 Feb 2020 18:36:36 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 |
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: address@hidden # v4.2
> Fixes: 7a3f542fbd
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
> 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 <address@hidden>
(block/io.c is Stefan’s department. :-))
Max
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