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

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

[Stefan Hajnoczi]

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);
         if (bytes <= 0) {
             return 0;
         }
-- 
2.43.0

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

[Philippe Mathieu-Daudé]

On 30/1/24 01:27, 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);
>           if (bytes <= 0) {
>               return 0;
>           }

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

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

[Stefan Hajnoczi]

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On Mon, Jan 29, 2024 at 07:27:12PM -0500, 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(-)

Thanks, applied to my block tree:
https://gitlab.com/stefanha/qemu/commits/block

Stefan

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

[Michael Tokarev]

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?

Thanks,

/mjt

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

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

[Michael Tokarev]

09.02.2024 00:21, Stefan Hajnoczi wrote:
> On Thu, 1 Feb 2024 at 06:37, Michael Tokarev <mjt@tls.msk.ru> wrote:

>>>        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.

Yeah, that's my thoughts exactly.  Also, most of the time, the cap will
be `size' anyway, not MAX.  Still thought I'd ask :)

Thank you for the confirmation!

/mjt

> 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.