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Tue, 02 Jun 2026 17:15:22 -0700 (PDT) From: Woojin Ji To: yonghong.song@linux.dev Cc: andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, daniel@iogearbox.net, eddyz87@gmail.com, jolsa@kernel.org, kuba@kernel.org, linux-kernel@vger.kernel.org, martin.lau@linux.dev, memxor@gmail.com, qmo@kernel.org, random6.xyz@gmail.com, sdf@fomichev.me, song@kernel.org Subject: Re: [PATCH bpf] bpftool: Use libbpf error code for flow dissector query Date: Wed, 3 Jun 2026 09:15:12 +0900 Message-ID: <20260603001512.19824-1-random6.xyz@gmail.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <22ab9375-e7e6-4929-a80c-0ee9cc74a907@linux.dev> References: <20260602170350.261035-1-random6.xyz@gmail.com> <18dda63b-79d0-47ad-b60b-82436ae0a017@linux.dev> <22ab9375-e7e6-4929-a80c-0ee9cc74a907@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Thanks for taking a look. Yes, the fd is opened read-only, so I do not mean to claim that close() on /proc/self/ns/net commonly fails in normal use, or that delayed writeback errors are expected here. The fragile part is that the BPF_PROG_QUERY error is consumed only after an intervening close(), even though bpf_prog_query() has already returned the negative errno in err. If close() changes errno, the old-kernel -EINVAL compatibility case can be missed. I reproduced this with an LD_PRELOAD fault injector that forced BPF_PROG_QUERY for BPF_FLOW_DISSECTOR to fail with EINVAL, and then forced close() on the netns fd to fail with EIO. The unpatched bpftool reported: Error: can't query prog: Input/output error With this patch, the same injected failure is handled as the intended non-fatal EINVAL compatibility case. I'll send a v2 with the commit message updated to make this clearer. The code change itself is unchanged.