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Re: [Qemu-block] [PATCH v2 2/2] block: fix write with zero flag set and

From: Alberto Garcia
Subject: Re: [Qemu-block] [PATCH v2 2/2] block: fix write with zero flag set and iovector provided
Date: Mon, 12 Feb 2018 16:03:49 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Mon 12 Feb 2018 02:14:01 PM CET, Anton Nefedov wrote:
> The normal bdrv_co_pwritev() use is either
>   - BDRV_REQ_ZERO_WRITE clear and iovector provided
>   - BDRV_REQ_ZERO_WRITE set and iovector == NULL
> while
>   - the flag clear and iovector == NULL is an assertion failure
>     in bdrv_co_do_zero_pwritev()
>   - the flag set and iovector provided is in fact allowed
>     (the flag prevails and zeroes are written)
> However the alignment logic does not support the latter case so the
> padding areas get overwritten with zeroes.

Oh, so this doesn't simply write zeroes in [offset, offset+bytes), but
also in the head and tail areas, instead of keeping the previous

This is a pretty serious bug, but I assume it can't be triggered
(bdrv_pwrite_zeroes() is used in complete clusters). Did you check if
there was any other scenario where this could happen?

> Signed-off-by: Anton Nefedov <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>

Reviewed-by: Alberto Garcia <address@hidden>


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