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Message-ID: <a71e0909-dc4c-43d7-88b2-8e92df89b386@gmail.com>
Date: Mon, 14 Oct 2024 11:14:53 +0100
From: "Colin King (gmail)" <colin.i.king@...il.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Jason Wang <jasowang@...hat.com>,
Paolo Abeni <pabeni@...hat.com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: virtio_net: support device stats
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@...ux.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@...hat.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@...il.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;
> };
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