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Tue, 21 Apr 2026 01:49:24 -0700 (PDT) Received: from [192.168.88.32] ([150.228.25.104]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-36141869e88sm12675045a91.1.2026.04.21.01.49.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Apr 2026 01:49:24 -0700 (PDT) Message-ID: Date: Tue, 21 Apr 2026 10:49:12 +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 v4 1/2] net/sched: taprio: fix NULL pointer dereference in class dump To: Weiming Shi , jhs@mojatatu.com, vinicius.gomes@intel.com, jiri@resnulli.us, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, shuah@kernel.org Cc: horms@kernel.org, vladimir.oltean@nxp.com, xmei5@asu.edu, netdev@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260416185501.647884-2-bestswngs@gmail.com> <20260416185501.647884-3-bestswngs@gmail.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20260416185501.647884-3-bestswngs@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/16/26 8:55 PM, Weiming Shi wrote: > @@ -2196,14 +2199,14 @@ static int taprio_graft(struct Qdisc *sch, unsigned long cl, > *old = q->qdiscs[cl - 1]; > if (FULL_OFFLOAD_IS_ENABLED(q->flags)) { > WARN_ON_ONCE(dev_graft_qdisc(dev_queue, new) != *old); > - if (new) > + if (new != &noop_qdisc) > qdisc_refcount_inc(new); > if (*old) > qdisc_put(*old); Unless I'm lost, it looks like taprio_leaf() can now return `noop_qdisc`. As a consequence, `old` can be a valid qdisc, NULL or even `noop_qdisc`. In the latter case it should not decrease the refcount, as it was not increased previously. /P