Re: [PATCH] pflash: fix sectors vs bytes confusion in blk_pread_nonzeroes()

[Stefan Hajnoczi]

On Thu, 1 Feb 2024 at 06:37, Michael Tokarev <mjt@tls.msk.ru> wrote:
>
> 30.01.2024 03:27, Stefan Hajnoczi wrote:
> > The following expression is incorrect because blk_pread_nonzeroes()
> > deals in units of bytes, not sectors:
> >
> >    bytes = MIN(size - offset, BDRV_REQUEST_MAX_SECTORS)
> >                                                ^^^^^^^
> >
> > BDRV_REQUEST_MAX_BYTES is the appropriate constant.
> >
> > Fixes: a4b15a8b9ef2 ("pflash: Only read non-zero parts of backend image")
> > Cc: Xiang Zheng <zhengxiang9@huawei.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >   hw/block/block.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/block/block.c b/hw/block/block.c
> > index 9f52ee6e72..ff503002aa 100644
> > --- a/hw/block/block.c
> > +++ b/hw/block/block.c
> > @@ -30,7 +30,7 @@ static int blk_pread_nonzeroes(BlockBackend *blk, hwaddr 
> > size, void *buf)
> >       BlockDriverState *bs = blk_bs(blk);
> >
> >       for (;;) {
> > -        bytes = MIN(size - offset, BDRV_REQUEST_MAX_SECTORS);
> > +        bytes = MIN(size - offset, BDRV_REQUEST_MAX_BYTES);
>
> Hmm.  This smells like a -stable material, but you know better not
> to Cc: qemu-stable@ for unrelated stuff...  Is it not for stable?

This is not a user-visible bug. The code still works with the smaller
MAX_SECTORS value thanks to the loop.

It doesn't hurt to include it in -stable but I also think it doesn't
help :-). It's just an inconsistency in the code.

Stefan