Re: virtio_net: support device stats

["Colin King (gmail)" <colin.i.king@gmail.com>]

On 14/10/2024 10:47, Michael S. Tsirkin wrote:
> On Mon, Oct 14, 2024 at 10:39:26AM +0100, Colin King (gmail) wrote:
>> Hi,
>>
>> Static analysis on Linux-next has detected a potential issue with the
>> following commit:
>>
>> commit 941168f8b40e50518a3bc6ce770a7062a5d99230
>> Author: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
>> Date:   Fri Apr 26 11:39:24 2024 +0800
>>
>>      virtio_net: support device stats
>>
>>
>> The issue is in function virtnet_stats_ctx_init, in drivers/net/virtio_net.c
>> as follows:
>>
>>          if (vi->device_stats_cap & VIRTIO_NET_STATS_TYPE_CVQ) {
>>                  queue_type = VIRTNET_Q_TYPE_CQ;
>>
>>                  ctx->bitmap[queue_type]   |= VIRTIO_NET_STATS_TYPE_CVQ;
>>                  ctx->desc_num[queue_type] +=
>> ARRAY_SIZE(virtnet_stats_cvq_desc);
>>                  ctx->size[queue_type]     += sizeof(struct
>> virtio_net_stats_cvq);
>>          }
>>
>>
>> ctx->bitmap is declared as a u32 however it is being bit-wise or'd with
>> VIRTIO_NET_STATS_TYPE_CVQ and this is defined as 1 << 32:
>>
>> include/uapi/linux/virtio_net.h:#define VIRTIO_NET_STATS_TYPE_CVQ (1ULL <<
>> 32)
>>
>> ..and hence the bit-wise or operation won't set any bits in ctx->bitmap
>> because 1ULL < 32 is too wide for a u32.
> 
> Indeed. Xuan Zhuo how did you test this patch?
> 
>> I suspect ctx->bitmap should be
>> declared as u64.
>>
>> Colin
>>
>>
> 
> In fact, it is read into a u64:
> 
>         u64 offset, bitmap;
> ....
>          bitmap = ctx->bitmap[queue_type];
> 
> we'll have to reorder fields to avoid wasting memory.
> Like this I guess:
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> Colin, can you confirm pls?

Fix looks sane to be, with u64 bitmap[3] struct size field re-ordering 
does not seem to make any difference on x86-64 (64 bytes) and i586 (56 
bytes) when I compiled with gcc-12, gcc-14 and clang-20.

I can't functionally test this though (not sure how).

Reviewed-by: Colin Ian King <colin.king@gmail.com>

> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c6af18948092..ef221429f784 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -4111,12 +4111,12 @@ struct virtnet_stats_ctx {
>   	/* Used to calculate the offset inside the output buffer. */
>   	u32 desc_num[3];
>   
> -	/* The actual supported stat types. */
> -	u32 bitmap[3];
> -
>   	/* Used to calculate the reply buffer size. */
>   	u32 size[3];
>   
> +	/* The actual supported stat types. */
> +	u64 bitmap[3];
> +
>   	/* Record the output buffer. */
>   	u64 *data;
>   };