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Tue, 21 Apr 2026 15:20:13 -0700 (PDT) Date: Tue, 21 Apr 2026 15:20:13 -0700 From: Stanislav Fomichev To: Jason Xing Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing Subject: Re: [PATCH net v2 1/8] xsk: reject sw-csum UMEM binding to IFF_TX_SKB_NO_LINEAR devices Message-ID: References: <20260420082805.14844-2-kerneljasonxing@gmail.com> 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-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On 04/21, Jason Xing wrote: > On Tue, Apr 21, 2026 at 3:34 AM Stanislav Fomichev wrote: > > > > > From: Jason Xing > > > > > > skb_checksum_help() is a common helper that writes the folded > > > 16-bit checksum back via skb->data + csum_start + csum_offset, > > > i.e. it relies on the skb's linear head and fails (with WARN_ONCE > > > and -EINVAL) when skb_headlen() is 0. > > > > > > AF_XDP generic xmit takes two very different paths depending on the > > > netdev. Drivers that advertise IFF_TX_SKB_NO_LINEAR (e.g. virtio_net) > > > skip the "copy payload into a linear head" step on purpose as a > > > performance optimisation: xsk_build_skb_zerocopy() only attaches UMEM > > > pages as frags and never calls skb_put(), so skb_headlen() stays 0 > > > for the whole skb. For these skbs there is simply no linear area for > > > skb_checksum_help() to write the csum into - the sw-csum fallback is > > > structurally inapplicable. > > > > > > The patch tries to catch this and reject the combination with error at > > > setup time. Rejecting at bind() converts this silent per-packet failure > > > into a synchronous, actionable -EOPNOTSUPP at setup time. HW csum and > > > launch_time metadata on IFF_TX_SKB_NO_LINEAR drivers are unaffected > > > because they do not call skb_checksum_help(). > > > > > > Without the patch, every descriptor carrying 'XDP_TX_METADATA | > > > XDP_TXMD_FLAGS_CHECKSUM' produces: > > > 1) a WARN_ONCE "offset (N) >= skb_headlen() (0)" from skb_checksum_help(), > > > 2) sendmsg() returning -EINVAL without consuming the descriptor > > > (invalid_descs is not incremented), > > > 3) a wedged TX ring: __xsk_generic_xmit() does not advance the > > > consumer on non-EOVERFLOW errors, so the next sendmsg() re-reads > > > the same descriptor and re-hits the same WARN until the socket > > > is closed. > > > > > > Closes: https://lore.kernel.org/all/20260419045822.843BFC2BCAF@smtp.kernel.org/#t > > > Fixes: 30c3055f9c0d ("xsk: wrap generic metadata handling onto separate function") > > > Signed-off-by: Jason Xing > > > --- > > > net/xdp/xsk_buff_pool.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c > > > index 37b7a68b89b3..c2521b6547e3 100644 > > > --- a/net/xdp/xsk_buff_pool.c > > > +++ b/net/xdp/xsk_buff_pool.c > > > @@ -169,6 +169,9 @@ int xp_assign_dev(struct xsk_buff_pool *pool, > > > if (force_zc && force_copy) > > > return -EINVAL; > > > > > > + if (pool->tx_sw_csum && (netdev->priv_flags & IFF_TX_SKB_NO_LINEAR)) > > > + return -EOPNOTSUPP; > > > + > > > if (xsk_get_pool_from_qid(netdev, queue_id)) > > > return -EBUSY; > > > > > > -- > > > 2.41.3 > > > > > > > Wondering whether a better fixes tag is commit 11614723af26 ("xsk: Add option > > to calculate TX checksum in SW")? > > > > Acked-by: Stanislav Fomichev > > Thanks for the check. But not really. It is the commit 30c3055f9c0d > that brings the csum support of IFF_TX_SKB_NO_LINEAR case where this > issue can be triggered (because this mode no longer puts data into skb > linear area). Ah, right, good point, it's the IFF_TX_SKB_NO_LINEAR path.