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Thu, 11 Jun 2026 01:30:36 -0700 (PDT) Received: from alrua-x1.borgediget.toke.dk ([45.145.92.2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-490e2c9057bsm55175325e9.5.2026.06.11.01.30.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Jun 2026 01:30:35 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 0F0E17BCC96; Thu, 11 Jun 2026 10:30:35 +0200 (CEST) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Sun Jian , bpf@vger.kernel.org Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, davem@davemloft.net, kuba@kernel.org, hawk@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, shuah@kernel.org, jiayuan.chen@linux.dev, menglong.dong@linux.dev, emil@etsalapatis.com, Sun Jian Subject: Re: [PATCH bpf v4 1/2] bpf: Run generic devmap egress prog on private skb In-Reply-To: <20260611080850.536996-2-sun.jian.kdev@gmail.com> References: <20260611080850.536996-1-sun.jian.kdev@gmail.com> <20260611080850.536996-2-sun.jian.kdev@gmail.com> X-Clacks-Overhead: GNU Terry Pratchett Date: Thu, 11 Jun 2026 10:30:35 +0200 Message-ID: <87se6t8ono.fsf@toke.dk> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Sun Jian writes: > Generic XDP devmap multi redirect uses skb_clone() for intermediate > destinations and sends the last destination with the original skb. This > can leave multiple destinations sharing the same packet data. > > This becomes visible after generic devmap egress-program support was > added: a devmap egress program may mutate packet data, and another > destination sharing the same data can observe that mutation. > > Native XDP broadcast redirect does not have this issue because > xdpf_clone() copies the frame data for each destination. Generic XDP > should provide the same per-destination isolation before running a > devmap egress program. > > Fix this by making cloned skbs private before running the generic devmap > egress program. Use skb_copy() instead of skb_unshare() so allocation > failure does not consume the skb and the existing caller error paths keep > their ownership semantics. > > Fixes: 2ea5eabaf04a ("bpf: devmap: Implement devmap prog execution for ge= neric XDP") > Suggested-by: Jiayuan Chen > Suggested-by: Jakub Kicinski > Reviewed-by: Toke H=C3=B8iland-J=C3=B8rgensen > Signed-off-by: Sun Jian > --- > kernel/bpf/devmap.c | 41 +++++++++++++++++++++++++++++++---------- > 1 file changed, 31 insertions(+), 10 deletions(-) > > diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c > index cc0a43ebab6b..a3d6c60dbddb 100644 > --- a/kernel/bpf/devmap.c > +++ b/kernel/bpf/devmap.c > @@ -512,35 +512,52 @@ static inline int __xdp_enqueue(struct net_device *= dev, struct xdp_frame *xdpf, > return 0; > } >=20=20 > -static u32 dev_map_bpf_prog_run_skb(struct sk_buff *skb, struct bpf_dtab= _netdev *dst) > +static int dev_map_bpf_prog_run_skb(struct sk_buff **pskb, > + struct bpf_dtab_netdev *dst, > + u32 *act) > { > + struct sk_buff *skb =3D *pskb; > struct xdp_txq_info txq =3D { .dev =3D dst->dev }; > struct xdp_buff xdp; > - u32 act; >=20=20 > - if (!dst->xdp_prog) > - return XDP_PASS; > + if (!dst->xdp_prog) { > + *act =3D XDP_PASS; > + return 0; > + } > + > + if (skb_cloned(skb)) { > + struct sk_buff *nskb; > + > + nskb =3D skb_copy(skb, GFP_ATOMIC); > + if (!nskb) > + return -ENOMEM; > + > + nskb->mac_len =3D skb->mac_len; > + consume_skb(skb); > + skb =3D nskb; > + *pskb =3D nskb; > + } So with all this pointer soup and back and forth, the version you had in v2 (with the check and skb_copy() in dev_map_generic_redirect()) was much cleaner :/ -Toke