dma-helpers: Fix iovec alignment

Commit 99868af3d0 changed the hardcoded constant BDRV_SECTOR_SIZE to a
dynamic field 'align' but introduced a bug. qemu_iovec_discard_back()
is now passed the wanted iov length instead of the actually required
amount that should be removed from the end of the iov.

The bug can likely only be hit in uncommon configurations, e.g. with
icount enabled or when reading from disk directly to device memory.

Fixes: 99868af3d0a75cf6 ("dma-helpers: explicitly pass alignment into DMA helpers")
Signed-off-by: Stefan Fritsch <sf@sfritsch.de>
---
 system/dma-helpers.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

On 12/4/24 10:06, Stefan Fritsch wrote:
> Commit 99868af3d0 changed the hardcoded constant BDRV_SECTOR_SIZE to a
> dynamic field 'align' but introduced a bug. qemu_iovec_discard_back()
> is now passed the wanted iov length instead of the actually required
> amount that should be removed from the end of the iov.
> 
> The bug can likely only be hit in uncommon configurations, e.g. with
> icount enabled or when reading from disk directly to device memory.
> 
> Fixes: 99868af3d0a75cf6 ("dma-helpers: explicitly pass alignment into DMA helpers")

Cc'ing developers mentioned in this commit.

> Signed-off-by: Stefan Fritsch <sf@sfritsch.de>
> ---
>   system/dma-helpers.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/system/dma-helpers.c b/system/dma-helpers.c
> index 9b221cf94e..c9677fd39b 100644
> --- a/system/dma-helpers.c
> +++ b/system/dma-helpers.c
> @@ -174,8 +174,7 @@ static void dma_blk_cb(void *opaque, int ret)
>       }
>   
>       if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
> -        qemu_iovec_discard_back(&dbs->iov,
> -                                QEMU_ALIGN_DOWN(dbs->iov.size, dbs->align));
> +        qemu_iovec_discard_back(&dbs->iov, dbs->iov.size % dbs->align);
>       }
>   
>       dbs->acb = dbs->io_func(dbs->offset, &dbs->iov,

On Fri, Apr 12, 2024 at 10:06:17AM +0200, Stefan Fritsch wrote:
> Commit 99868af3d0 changed the hardcoded constant BDRV_SECTOR_SIZE to a
> dynamic field 'align' but introduced a bug. qemu_iovec_discard_back()
> is now passed the wanted iov length instead of the actually required
> amount that should be removed from the end of the iov.
> 
> The bug can likely only be hit in uncommon configurations, e.g. with
> icount enabled or when reading from disk directly to device memory.
> 
> Fixes: 99868af3d0a75cf6 ("dma-helpers: explicitly pass alignment into DMA helpers")
> Signed-off-by: Stefan Fritsch <sf@sfritsch.de>
> ---
>  system/dma-helpers.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)

Wow, that bug has been latent for a while (2016, v2.8.0).  As such, I
don't think it's worth holding up 9.0 for, but it is definitely worth
cc'ing qemu-stable (done now).

> 
> diff --git a/system/dma-helpers.c b/system/dma-helpers.c
> index 9b221cf94e..c9677fd39b 100644
> --- a/system/dma-helpers.c
> +++ b/system/dma-helpers.c
> @@ -174,8 +174,7 @@ static void dma_blk_cb(void *opaque, int ret)
>      }
>  
>      if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
> -        qemu_iovec_discard_back(&dbs->iov,
> -                                QEMU_ALIGN_DOWN(dbs->iov.size, dbs->align));
> +        qemu_iovec_discard_back(&dbs->iov, dbs->iov.size % dbs->align);

Before the regression, it was:

-    if (dbs->iov.size & ~BDRV_SECTOR_MASK) {
-        qemu_iovec_discard_back(&dbs->iov, dbs->iov.size & ~BDRV_SECTOR_MASK);
+    if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
+        qemu_iovec_discard_back(&dbs->iov,

If dbs->align is always a power of two, we can use '& (dbs->align -
1)' to avoid a hardware division, to match the original code; bug
QEMU_IS_ALIGNED does not require a power of two, so your choice of '%
dbs->align' seems reasonable.

Reviewed-by: Eric Blake <eblake@redhat.com>

adding John Snow to CC because he investigated this in 2020.

On Fri, 12 Apr 2024, Eric Blake wrote:

> On Fri, Apr 12, 2024 at 10:06:17AM +0200, Stefan Fritsch wrote:
> > Commit 99868af3d0 changed the hardcoded constant BDRV_SECTOR_SIZE to a
> > dynamic field 'align' but introduced a bug. qemu_iovec_discard_back()
> > is now passed the wanted iov length instead of the actually required
> > amount that should be removed from the end of the iov.
> > 
> > The bug can likely only be hit in uncommon configurations, e.g. with
> > icount enabled or when reading from disk directly to device memory.
> > 
> > Fixes: 99868af3d0a75cf6 ("dma-helpers: explicitly pass alignment into DMA helpers")
> > Signed-off-by: Stefan Fritsch <sf@sfritsch.de>
> > ---
> >  system/dma-helpers.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> Wow, that bug has been latent for a while (2016, v2.8.0).  As such, I
> don't think it's worth holding up 9.0 for, but it is definitely worth
> cc'ing qemu-stable (done now).

we had some more internal discussions and did some more googling, and it 
turns out that this is actually part of a known issue:

https://lists.nongnu.org/archive/html/qemu-block/2020-07/msg01866.html
https://gitlab.com/qemu-project/qemu/-/issues/259

In the above mail to qemu-block, John Snow listed 4 problems in the code 
but my patch only fixes the first one. Considering that the code may also 
write data to the wrong place (problem 2), I wonder if an assert in the 
same place would be better until the underlying issues have been fixed?


> > 
> > diff --git a/system/dma-helpers.c b/system/dma-helpers.c
> > index 9b221cf94e..c9677fd39b 100644
> > --- a/system/dma-helpers.c
> > +++ b/system/dma-helpers.c
> > @@ -174,8 +174,7 @@ static void dma_blk_cb(void *opaque, int ret)
> >      }
> >  
> >      if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
> > -        qemu_iovec_discard_back(&dbs->iov,
> > -                                QEMU_ALIGN_DOWN(dbs->iov.size, dbs->align));
> > +        qemu_iovec_discard_back(&dbs->iov, dbs->iov.size % dbs->align);
> 
> Before the regression, it was:
> 
> -    if (dbs->iov.size & ~BDRV_SECTOR_MASK) {
> -        qemu_iovec_discard_back(&dbs->iov, dbs->iov.size & ~BDRV_SECTOR_MASK);
> +    if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
> +        qemu_iovec_discard_back(&dbs->iov,
> 
> If dbs->align is always a power of two, we can use '& (dbs->align -
> 1)' to avoid a hardware division, to match the original code; bug
> QEMU_IS_ALIGNED does not require a power of two, so your choice of '%
> dbs->align' seems reasonable.
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>
> 
>