Re: [Qemu-devel] [PATCH] block: fix write with zero flag set and iovector provided

[Eric Blake]

On 02/01/2018 08:16 AM, Anton Nefedov wrote:
> The normal bdrv_co_pwritev() use is either
>   - BDRV_REQ_ZERO_WRITE reset and iovector provided

s/reset/clear/

>   - BDRV_REQ_ZERO_WRITE set and iovector == NULL
> 
> while
>   - the flag reset and iovector == NULL is an assertion failure

again

>     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.
> 
> Solution could be to forbid such case or just use bdrv_co_do_zero_pwritev()
> alignment for it which also makes the code a bit more obvious anyway.
> 
> Signed-off-by: Anton Nefedov <address@hidden>
> ---
>  block/io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 7ea4023..cf63fd0 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1701,7 +1701,7 @@ int coroutine_fn bdrv_co_pwritev(BdrvChild *child,
>       */
>      tracked_request_begin(&req, bs, offset, bytes, BDRV_TRACKED_WRITE);
>  
> -    if (!qiov) {
> +    if (flags & BDRV_REQ_ZERO_WRITE) {
>          ret = bdrv_co_do_zero_pwritev(child, offset, bytes, flags, &req);

So now, the flag rules, but we assert that !qiov (so it would only break
a caller that passed the flag but used qiov, which you argued shouldn't
exist).

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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