netdev - Re: [PATCH net v3 2/2] net: gro: add flush check in udp_gro_receive

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Date: Wed, 24 Apr 2024 22:45:55 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Richard Gobert <richardbgobert@...il.com>, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 dsahern@...nel.org, 
 willemdebruijn.kernel@...il.com, 
 alobakin@...me, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org
Cc: Richard Gobert <richardbgobert@...il.com>
Subject: Re: [PATCH net v3 2/2] net: gro: add flush check in
 udp_gro_receive_segment

Richard Gobert wrote:
> GRO-GSO path is supposed to be transparent and as such L3 flush checks are
> relevant to all UDP flows merging in GRO. This patch uses the same logic
> and code from tcp_gro_receive, terminating merge if flush is non zero.
> 
> Fixes: 36707061d6ba ("udp: allow forwarding of plain (non-fraglisted) UDP GRO packets")

Should this be

Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")

>  net/ipv4/udp_offload.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index fd29d21d579c..8721fe5beca2 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -471,6 +471,7 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
>  	struct sk_buff *p;
>  	unsigned int ulen;
>  	int ret = 0;
> +	int flush;
>  
>  	/* requires non zero csum, for symmetry with GSO */
>  	if (!uh->check) {
> @@ -504,13 +505,22 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
>  			return p;
>  		}
>  
> +		flush = NAPI_GRO_CB(p)->flush;
> +
> +		if (NAPI_GRO_CB(p)->flush_id != 1 ||
> +		    NAPI_GRO_CB(p)->count != 1 ||
> +		    !NAPI_GRO_CB(p)->is_atomic)
> +			flush |= NAPI_GRO_CB(p)->flush_id;
> +		else
> +			NAPI_GRO_CB(p)->is_atomic = false;
> +
>  		/* Terminate the flow on len mismatch or if it grow "too much".
>  		 * Under small packet flood GRO count could elsewhere grow a lot
>  		 * leading to excessive truesize values.
>  		 * On len mismatch merge the first packet shorter than gso_size,
>  		 * otherwise complete the GRO packet.
>  		 */
> -		if (ulen > ntohs(uh2->len)) {
> +		if (ulen > ntohs(uh2->len) || flush) {
>  			pp = p;

I suppose this branch already could immediately return p. And avoid
a level of indentation below. But agreed to not change that in this
bug fix.