Re: [PATCH] udp: gso: fix MTU check for small packets

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Yan Zhai wrote:
> Commit 4094871db1d6 ("udp: only do GSO if # of segs > 1") avoided GSO
> for small packets. But the kernel currently dismisses GSO requests only
> after checking MTU on gso_size. This means any packets, regardless of
> their payload sizes, would be dropped when MTU is smaller than requested
> gso_size.

Is this a realistic concern? How did you encounter this in practice.

It *is* a misconfiguration to configure a gso_size larger than MTU.

> Meanwhile, EINVAL would be returned in this case, making it
> very misleading to debug.

Misleading is subjective. I'm not sure what is misleading here. From
my above comment, I believe this is correctly EINVAL.

That said, if this impacts a real workload we could reconsider
relaxing the check. I.e., allowing through packets even when an
application has clearly misconfigured UDP_SEGMENT.

> 
> Ideally, do not check any GSO related constraints when payload size is
> smaller than requested gso_size, and return EMSGSIZE on MTU check
> failure consistently for all packets to ease debugging.
> 
> Fixes: 4094871db1d6 ("udp: only do GSO if # of segs > 1")
> Signed-off-by: Yan Zhai <yan@cloudflare.com>
> ---
>  net/ipv4/udp.c                       | 18 ++++++++----------
>  net/ipv6/udp.c                       | 18 ++++++++----------
>  tools/testing/selftests/net/udpgso.c | 14 ++++++++++++++
>  3 files changed, 30 insertions(+), 20 deletions(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index c472c9a57cf6..9aed1b4a871f 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1137,13 +1137,13 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
>  	uh->len = htons(len);
>  	uh->check = 0;
>  
> -	if (cork->gso_size) {
> +	if (cork->gso_size && datalen > cork->gso_size) {
>  		const int hlen = skb_network_header_len(skb) +
>  				 sizeof(struct udphdr);
>  
>  		if (hlen + cork->gso_size > cork->fragsize) {
>  			kfree_skb(skb);
> -			return -EINVAL;
> +			return -EMSGSIZE;
>  		}
>  		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
>  			kfree_skb(skb);
> @@ -1158,15 +1158,13 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
>  			return -EIO;
>  		}
>  
> -		if (datalen > cork->gso_size) {
> -			skb_shinfo(skb)->gso_size = cork->gso_size;
> -			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> -			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> -								 cork->gso_size);
> +		skb_shinfo(skb)->gso_size = cork->gso_size;
> +		skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> +		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> +							 cork->gso_size);
>  
> -			/* Don't checksum the payload, skb will get segmented */
> -			goto csum_partial;
> -		}
> +		/* Don't checksum the payload, skb will get segmented */
> +		goto csum_partial;
>  	}
>  
>  	if (is_udplite)  				 /*     UDP-Lite      */
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 6671daa67f4f..6cdc8ce4c6f9 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1385,13 +1385,13 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
>  	uh->len = htons(len);
>  	uh->check = 0;
>  
> -	if (cork->gso_size) {
> +	if (cork->gso_size && datalen > cork->gso_size) {
>  		const int hlen = skb_network_header_len(skb) +
>  				 sizeof(struct udphdr);
>  
>  		if (hlen + cork->gso_size > cork->fragsize) {
>  			kfree_skb(skb);
> -			return -EINVAL;
> +			return -EMSGSIZE;
>  		}
>  		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
>  			kfree_skb(skb);
> @@ -1406,15 +1406,13 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
>  			return -EIO;
>  		}
>  
> -		if (datalen > cork->gso_size) {
> -			skb_shinfo(skb)->gso_size = cork->gso_size;
> -			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> -			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> -								 cork->gso_size);
> +		skb_shinfo(skb)->gso_size = cork->gso_size;
> +		skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> +		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> +							 cork->gso_size);
>  
> -			/* Don't checksum the payload, skb will get segmented */
> -			goto csum_partial;
> -		}
> +		/* Don't checksum the payload, skb will get segmented */
> +		goto csum_partial;
>  	}
>  
>  	if (is_udplite)
> diff --git a/tools/testing/selftests/net/udpgso.c b/tools/testing/selftests/net/udpgso.c
> index 3f2fca02fec5..fb73f1c331fb 100644
> --- a/tools/testing/selftests/net/udpgso.c
> +++ b/tools/testing/selftests/net/udpgso.c
> @@ -102,6 +102,13 @@ struct testcase testcases_v4[] = {
>  		.gso_len = CONST_MSS_V4,
>  		.r_num_mss = 1,
>  	},
> +	{
> +		/* datalen <= MSS < gso_len: will fall back to no GSO */
> +		.tlen = CONST_MSS_V4,
> +		.gso_len = CONST_MSS_V4 + 1,
> +		.r_num_mss = 0,
> +		.r_len_last = CONST_MSS_V4,
> +	},
>  	{
>  		/* send a single MSS + 1B */
>  		.tlen = CONST_MSS_V4 + 1,
> @@ -205,6 +212,13 @@ struct testcase testcases_v6[] = {
>  		.gso_len = CONST_MSS_V6,
>  		.r_num_mss = 1,
>  	},
> +	{
> +		/* datalen <= MSS < gso_len: will fall back to no GSO */
> +		.tlen = CONST_MSS_V6,
> +		.gso_len = CONST_MSS_V6 + 1,
> +		.r_num_mss = 0,
> +		.r_len_last = CONST_MSS_V6,
> +	},
>  	{
>  		/* send a single MSS + 1B */
>  		.tlen = CONST_MSS_V6 + 1,
> -- 
> 2.30.2
> 
>