Re: [PATCH] block/io: fix bdrv_co_do_copy_on_readv

[John Snow <jsnow@redhat.com>]

On 3/12/20 4:19 AM, Vladimir Sementsov-Ogievskiy wrote:
> Prior to 1143ec5ebf4 it was OK to qemu_iovec_from_buf() from aligned-up
> buffer to original qiov, as qemu_iovec_from_buf() will stop at qiov end
> anyway.
> 
> But after 1143ec5ebf4 we assume that bdrv_co_do_copy_on_readv works on
> part of original qiov, defined by qiov_offset and bytes. So we must not
> touch qiov behind qiov_offset+bytes bound. Fix it.
> 

For the purposes of the stable branch commit log, how does the bug
manifest? Are there known cases? What's the impact?

(Do we have tests?)

> Cc: qemu-stable@nongnu.org # v4.2
> Fixes: 1143ec5ebf4
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 7e4cb74cf4..aba67f66b9 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1399,7 +1399,7 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
>              if (!(flags & BDRV_REQ_PREFETCH)) {
>                  qemu_iovec_from_buf(qiov, qiov_offset + progress,
>                                      bounce_buffer + skip_bytes,
> -                                    pnum - skip_bytes);
> +                                    MIN(pnum - skip_bytes, bytes - progress));
>              }
>          } else if (!(flags & BDRV_REQ_PREFETCH)) {
>              /* Read directly into the destination */
> 
Even if I don't understand the bug, the tighter bound seems provably
correct anyway, so...

Reviewed-by: John Snow <jsnow@redhat.com>