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[34.86.60.15]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-51e4ea35b4bsm1445410e0c.6.2025.01.27.06.33.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Jan 2025 06:33:16 -0800 (PST) Date: Mon, 27 Jan 2025 09:33:16 -0500 From: Willem de Bruijn To: Yan Zhai , netdev@vger.kernel.org Cc: Willem de Bruijn , "David S. Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Shuah Khan , Josh Hunt , Alexander Duyck , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Message-ID: <6797992c28a23_3f1a294d6@willemb.c.googlers.com.notmuch> In-Reply-To: References: Subject: Re: [PATCH] udp: gso: fix MTU check for small packets Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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 > --- > 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 > >