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Thu, 30 Apr 2026 01:54:41 -0700 (PDT) Received: from [192.168.88.32] ([150.228.93.27]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48a81ed6bafsm83284215e9.2.2026.04.30.01.54.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Apr 2026 01:54:41 -0700 (PDT) Message-ID: <410c814a-399a-4eb9-a39a-d1e5fecd6b33@redhat.com> Date: Thu, 30 Apr 2026 10:54:39 +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] ppp: consolidate RX skb queueing To: Qingfang Deng , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Guillaume Nault , Breno Leitao , Taegu Ha , Kees Cook , Sebastian Andrzej Siewior , linux-ppp@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260428024426.48605-1-qingfang.deng@linux.dev> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20260428024426.48605-1-qingfang.deng@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/28/26 4:44 AM, Qingfang Deng wrote: > In ppp_input() and ppp_receive_nonmp_frame(), received skbs are queued > for userspace delivery using the same open-coded pattern: > > skb_queue_tail(&pf->rq, skb); > while (pf->rq.qlen > PPP_MAX_RQLEN && > (skb = skb_dequeue(&pf->rq))) > kfree_skb(skb); > wake_up_interruptible(&pf->rwait); > > This has a potential race: skb_queue_tail() releases the queue lock, > then qlen is read locklessly before skb_dequeue() re-acquires it. > Another CPU enqueueing concurrently could cause the length check to see > stale data. This race is benign, as it only causes extra skbs to be > freed in the worst case. > > Introduce ppp_file_queue_rx_skb() to perform the enqueue, length check, > and trim atomically under a single pf->rq.lock critical section. As both > callers have softirq disabled, plain spin_lock() can be used instead of > _bh()/_irqsave() variants. Since only one skb is enqueued at a time, the > queue can exceed PPP_MAX_RQLEN by at most one frame, so replace the > while-loop with an if-statement. While at it, use skb_queue_len() > instead of open-coding the qlen access. > > Signed-off-by: Qingfang Deng > --- > drivers/net/ppp/ppp_generic.c | 37 ++++++++++++++++++++++------------- > 1 file changed, 23 insertions(+), 14 deletions(-) > > diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c > index 57c68efa5ff8..6ab5011540a0 100644 > --- a/drivers/net/ppp/ppp_generic.c > +++ b/drivers/net/ppp/ppp_generic.c > @@ -2307,6 +2307,27 @@ static bool ppp_channel_bridge_input(struct channel *pch, struct sk_buff *skb) > return !!pchb; > } > > +/* Queue up and deliver a received skb to userspace. > + * Must be called in softirq. > + */ > +static void ppp_file_queue_rx_skb(struct ppp_file *pf, struct sk_buff *skb) > +{ > + spin_lock(&pf->rq.lock); > + __skb_queue_tail(&pf->rq, skb); > + /* limit queue length by dropping old frames */ > + if (unlikely(skb_queue_len(&pf->rq) > PPP_MAX_RQLEN)) { > + struct sk_buff *old = __skb_peek(&pf->rq); > + > + __skb_unlink(old, &pf->rq); > + spin_unlock(&pf->rq.lock); > + kfree_skb(old); > + } else { > + spin_unlock(&pf->rq.lock); Note that after __skb_queue_tail(), skb_queue_len(&pf->rq) could be == PPP_MAX_RQLEN + 2, due to the slightly different check in ppp_prepare_tx_skb(). I think the above it could/should be simplified to: while (unlikely(skb_queue_len(&pf->rq) > PPP_MAX_RQLEN)) kfree_skb(__skb_dequeue(&pf->rq)); spin_unlock(&pf->rq.lock); And possibly it would make sense to consolidate the test in ppp_prepare_tx_skb(), too for consistency - in that case an `if` statement should become enough. /P