linux-kernel - Re: virtio_net: support device stats

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Message-ID: <20241014054305-mutt-send-email-mst@kernel.org>
Date: Mon, 14 Oct 2024 05:47:41 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: "Colin King (gmail)" <colin.i.king@...il.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 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?

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;
 };
-- 
MST