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Sun, 14 Jun 2026 04:19:50 -0700 (PDT) Received: from [192.168.88.32] ([150.228.25.72]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-492202edf89sm165147515e9.1.2026.06.14.04.19.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 14 Jun 2026 04:19:49 -0700 (PDT) Message-ID: <554ff2bd-e4d7-4e64-8ec4-86dc7da85992@redhat.com> Date: Sun, 14 Jun 2026 13:19:47 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v7 04/11] net: Enable BIG TCP with partial GSO To: Alice Mikityanska , Daniel Borkmann , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Xin Long , Willem de Bruijn , Willem de Bruijn , David Ahern , Nikolay Aleksandrov Cc: Shuah Khan , Stanislav Fomichev , Andrew Lunn , Simon Horman , Florian Westphal , netdev@vger.kernel.org, Alice Mikityanska References: <20260611192955.604661-1-alice.kernel@fastmail.im> <20260611192955.604661-5-alice.kernel@fastmail.im> From: Paolo Abeni Content-Language: en-US In-Reply-To: <20260611192955.604661-5-alice.kernel@fastmail.im> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/11/26 9:29 PM, Alice Mikityanska wrote: > From: Alice Mikityanska > > skb_segment is called for partial GSO, when netif_needs_gso returns true > in validate_xmit_skb. Partial GSO is needed, for example, when > segmentation of tunneled traffic is offloaded to a NIC that only > supports inner checksum offload. > > Currently, skb_segment clamps the segment length to 65534 bytes, because > gso_size == 65535 is a special value GSO_BY_FRAGS, and we don't want > to accidentally assign mss = 65535, as it would fall into the > GSO_BY_FRAGS check further in the function. > > This implementation, however, artificially blocks len > 65534, which is > possible since the introduction of BIG TCP. To allow bigger lengths and > avoid resegmentation of BIG TCP packets, store the gso_by_frags flag in > the beginning and don't use a special value of mss for this purpose > after mss was modified. > > Signed-off-by: Alice Mikityanska > Reviewed-by: Willem de Bruijn > --- > drivers/net/netdevsim/psp.c | 2 +- > net/core/skbuff.c | 10 +++++----- > 2 files changed, 6 insertions(+), 6 deletions(-) > > diff --git a/drivers/net/netdevsim/psp.c b/drivers/net/netdevsim/psp.c > index d3e36c74be62..6b3532b5e360 100644 > --- a/drivers/net/netdevsim/psp.c > +++ b/drivers/net/netdevsim/psp.c > @@ -92,7 +92,7 @@ nsim_do_psp(struct sk_buff *skb, struct netdevsim *ns, > * provide a valid checksum here, so the skb isn't dropped. > */ > uh = udp_hdr(skb); > - udplen = ntohs(uh->len) ?: skb->len - skb_transport_offset(skb); > + udplen = udp_get_len(skb, uh, skb_transport_offset(skb)); > csum = skb_checksum(skb, skb_transport_offset(skb), > udplen, 0); > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > index c64693fcb2d1..5dcee79df8cf 100644 > --- a/net/core/skbuff.c > +++ b/net/core/skbuff.c > @@ -4773,6 +4773,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > struct sk_buff *tail = NULL; > struct sk_buff *list_skb = skb_shinfo(head_skb)->frag_list; > unsigned int mss = skb_shinfo(head_skb)->gso_size; > + bool gso_by_frags = mss == GSO_BY_FRAGS; > unsigned int doffset = head_skb->data - skb_mac_header(head_skb); > unsigned int offset = doffset; > unsigned int tnl_hlen = skb_tnl_header_len(head_skb); > @@ -4788,7 +4789,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > int nfrags, pos; > > if ((skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY) && > - mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) { > + !gso_by_frags && mss != skb_headlen(head_skb)) { > struct sk_buff *check_skb; > > for (check_skb = list_skb; check_skb; check_skb = check_skb->next) { > @@ -4816,7 +4817,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > sg = !!(features & NETIF_F_SG); > csum = !!can_checksum_protocol(features, proto); > > - if (sg && csum && (mss != GSO_BY_FRAGS)) { > + if (sg && csum && !gso_by_frags) { > if (!(features & NETIF_F_GSO_PARTIAL)) { > struct sk_buff *iter; > unsigned int frag_len; > @@ -4850,9 +4851,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > /* GSO partial only requires that we trim off any excess that > * doesn't fit into an MSS sized block, so take care of that > * now. > - * Cap len to not accidentally hit GSO_BY_FRAGS. > */ > - partial_segs = min(len, GSO_BY_FRAGS - 1) / mss; > + partial_segs = len / mss; Sashiko/gemini says the above can lead to hit BUG_ON() later. I *think* it's not a false positive, as it looks like skb_segment() assumes an skb can hold `mss` bytes without resorting to frag_list usage, and mss > MAX_SKB_FRAGS * PAGE_SIZE breaks such assumption. I think handling correctly this case will requires some non trivial surgery to skb_segment: both `while (pos < offset + len) {` loops must be updated to feed data from `frags` as needed instead of BUG_ON()/net_warn_ratelimited(skb_shinfo(nskb)->nr_frags >= MAX_SKB_FRAGS); /P