Linux Virtualization - Aug 2008 - [PATCH] virtio_balloon: fix towards_target when deflating balloon

Both v and vb->num_pages are u32 and unsigned int respectively.  If v is less
than vb->num_pages (and it is, when deflating the balloon), the result is a
very large 32-bit number.  Since we're returning a s64, instead of getting
the
same negative number we desire, we get a very large positive number.

This handles the case where v < vb->num_pages and ensures we get a small,
negative, s64 as the result.

Rusty: please push this for 2.6.27-rc4.  It's probably appropriate for the
stable tree too as it will cause an unexpected OOM when ballooning.

Signed-off-by: Anthony Liguori <aliguori at us.ibm.com>

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index bfef604..bd3c384 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -158,7 +158,10 @@ static inline s64 towards_target(struct virtio_balloon *vb)
 	vb->vdev->config->get(vb->vdev,
 			      offsetof(struct virtio_balloon_config, num_pages),
 			      &v, sizeof(v));
-	return v - vb->num_pages;
+	if (v < vb->num_pages)
+		return -(s64)(vb->num_pages - v);
+	else
+		return v - vb->num_pages;
 }
 
 static void update_balloon_size(struct virtio_balloon *vb)

On Mon, 18 Aug 2008, Anthony Liguori wrote: <
> 
> This handles the case where v < vb->num_pages and ensures we get a 
> small, negative, s64 as the result.
That's just horrible code.

Maybe the compiler notices that you're doing something stupid, but 
basically, please don't do this.
> -	return v - vb->num_pages;
> +	if (v < vb->num_pages)
> +		return -(s64)(vb->num_pages - v);
> +	else
> +		return v - vb->num_pages;
What's wrong with just doing

	return (s64)v - vb->num_pages;

instead?

Casting 'v' to s64 guarantees that the subtraction will eb done in 64 
bits, and the compiler can just generate the trivial non-conditional code.

		Linus

On Tuesday 19 August 2008 08:15:31 Anthony Liguori
wrote:
> -	return v - vb->num_pages;
> +	if (v < vb->num_pages)
> +		return -(s64)(vb->num_pages - v);
> +	else
> +		return v - vb->num_pages;
With all due respect, WTF?

Did you mean:

	return (s64)v - vb->num_pages;

I'm really amazed this bug got this far though...
Rusty.