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[2a0c:4d80:42:443::2]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43b64717e97sm41173985f8f.35.2026.03.24.09.18.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Mar 2026 09:18:55 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 24AE35A470F; Tue, 24 Mar 2026 17:18:54 +0100 (CET) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Leon Hwang , bpf@vger.kernel.org Cc: Alexei Starovoitov , Daniel Borkmann , John Fastabend , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Shuah Khan , "David S . Miller" , Jakub Kicinski , Jesper Dangaard Brouer , Leon Hwang , Lorenzo Bianconi , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, netdev@vger.kernel.org, kernel-patches-bot@fb.com Subject: Re: [PATCH bpf-next 1/3] bpf: Disallow freplace on XDP with mismatched xdp_has_frags values In-Reply-To: <20260324150444.68166-2-leon.hwang@linux.dev> References: <20260324150444.68166-1-leon.hwang@linux.dev> <20260324150444.68166-2-leon.hwang@linux.dev> X-Clacks-Overhead: GNU Terry Pratchett Date: Tue, 24 Mar 2026 17:18:54 +0100 Message-ID: <87ecl9mc9d.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 Leon Hwang writes: > xdp_has_frags was introduced by the commit > c2f2cdbeffda ("bpf: introduce BPF_F_XDP_HAS_FRAGS flag in prog_flags loading the ebpf program"). > > The commit f45d5b6ce2e8 ("bpf: generalise tail call map compatibility check") > was to ensure backwards compatibility against tail calls. However, it > missed that XDP progs can be extended by freplace progs, which could break > the backwards compatibility, e.g. xdp_has_frags=true freplace progs are > allowed to attach to xdp_has_frags=false XDP progs. > > To avoid breaking the backwards compatibility via freplace, disallow > freplace on XDP programs with different xdp_has_frags values. The problem you describe is not actually a problem, though? A frags-aware program can run on a non-frags interface just fine. You're messing with long-standing behaviour (since 5.18!) to solve a non-existent problem. In a way that completely breaks the frags handling in libxdp[0]: Running tests from ./test-libxdp.sh [test_link_so] PASS [test_link_a] PASS [test_old_dispatcher] PASS [test_xdp_devbound] PASS [test_xdp_frags] FAIL Kernel supports XDP programs with frags check_load_frags: FAILED check_load_nofrags_success: PASSED check_load_nofrags_fail: PASSED check_load_frags_multi: FAILED check_load_mix_big: FAILED check_load_mix_small: FAILED Test test_xdp_frags exited with return code: 1 Please don't do that. -Toke [0] Run 'make test' here: https://github.com/xdp-project/xdp-tools