Subject: Re: [PATCH] tun: Use netif_receive_skb instead of netif_rx

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 29 Nov 2016 18:20:22 +0200

On Tue, Nov 29, 2016 at 04:25:36PM +0100, Andrey Konovalov wrote:
> This patch changes tun.c to call netif_receive_skb instead of netif_rx
> when a packet is received. The difference between the two is that netif_rx
> queues the packet into the backlog, and netif_receive_skb proccesses the
> packet in the current context.
> 
> This patch is required for syzkaller [1] to collect coverage from packet
> receive paths, when a packet being received through tun (syzkaller collects
> coverage per process in the process context).
> 
> A similar patch was introduced back in 2010 [2, 3], but the author found
> out that the patch doesn't help with the task he had in mind (for cgroups
> to shape network traffic based on the original process) and decided not to
> go further with it. The main concern back then was about possible stack
> exhaustion with 4K stacks, but CONFIG_4KSTACKS was removed and stacks are
> 8K now.
> 
> [1] https://github.com/google/syzkaller
> 
> [2] https://www.spinics.net/lists/netdev/thrd440.html#130570
> 
> [3] https://www.spinics.net/lists/netdev/msg130570.html
> 
> Signed-off-by: Andrey Konovalov <[email-masked]>

This was on my list of things to investigate ever since
8k stack default went in. Thanks for looking into this!

I note that there are still seem to exit 3 architectures that do have
CONFIG_4KSTACKS.

How about a wrapper that does netif_rx_ni with CONFIG_4KSTACKS.

> ---
>  drivers/net/tun.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 8093e39..4b56e91 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1304,7 +1304,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  	skb_probe_transport_header(skb, 0);
>  
>  	rxhash = skb_get_hash(skb);
> -	netif_rx_ni(skb);
> +	local_bh_disable();
> +	netif_receive_skb(skb);
> +	local_bh_enable();
>  
>  	stats = get_cpu_ptr(tun->pcpu_stats);
>  	u64_stats_update_begin(&stats->syncp);
> -- 
> 2.8.0.rc3.226.g39d4020