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Fri, 15 May 2026 09:41:24 -0700 (PDT) From: Aaron Esau To: imv4bel@gmail.com Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, sultan@kerneltoast.com, sd@queasysnail.net, steffen.klassert@secunet.com, herbert@gondor.apana.org.au, dsahern@kernel.org, netdev@vger.kernel.org, stable@vger.kernel.org, Aaron Esau Subject: Re: [PATCH net v4] net: skbuff: propagate shared-frag marker through frag-transfer helpers Date: Fri, 15 May 2026 11:41:21 -0500 Message-ID: <20260515164121.2608076-1-aaron1esau@gmail.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit skb_segment() propagates SKBFL_SHARED_FRAG from head_skb only. When segments pull frags from frag_list members, the flag is never propagated from those members into the segment skb. There are two miss sites: 1. Line ~4986: a new nskb propagates only from head_skb, but frag_skb may already point to a list_skb carried over from the previous segment's iteration (i, nfrags, frag_skb persist across the outer do/while). 2. When the inner loop exhausts head_skb frags and switches to a list_skb (line ~4999-5002), frag_skb is updated but its SKBFL_SHARED_FRAG is not propagated into nskb. Your v4 GRO fix means head_skb will normally carry the flag, so skb_segment() picks it up indirectly. But skb_segment() itself should propagate from frag_list members directly --- otherwise any non-GRO frag_list producer re-exposes the gap. diff --git a/net/core/skbuff.c b/net/core/skbuff.c --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -4986,7 +4986,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, - skb_shinfo(nskb)->flags |= skb_shinfo(head_skb)->flags & - SKBFL_SHARED_FRAG; + skb_shinfo(nskb)->flags |= (skb_shinfo(head_skb)->flags | + skb_shinfo(frag_skb)->flags) & + SKBFL_SHARED_FRAG; if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC)) @@ -5000,6 +5001,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, frag = skb_shinfo(list_skb)->frags; frag_skb = list_skb; + skb_shinfo(nskb)->flags |= skb_shinfo(frag_skb)->flags & SKBFL_SHARED_FRAG; + if (!skb_headlen(list_skb)) { BUG_ON(!nfrags); } else { Site 1 covers segments that start mid-list_skb (frag_skb carried from the previous segment). Site 2 covers segments that switch from head_skb frags to list_skb frags mid-construction. Fixes: cef401de7be8 ("net: fix possible wrong checksum generation")