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([2620:10d:c090:500::7:f7b9]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29498d0827fsm121791055ad.31.2025.10.28.10.18.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Oct 2025 10:18:48 -0700 (PDT) Message-ID: Subject: Re: [PATCH bpf-next v2 1/2] bpf: Fix tnum_overlap to check for zero mask intersection From: Eduard Zingerman To: bot+bpf-ci@kernel.org, kafai.wan@linux.dev, ast@kernel.org, daniel@iogearbox.net, john.fastabend@gmail.com, andrii@kernel.org, martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, shuah@kernel.org, paul.chaignon@gmail.com, m.shachnai@gmail.com, harishankar.vishwanathan@gmail.com, colin.i.king@gmail.com, luis.gerhorst@fau.de, shung-hsi.yu@suse.com, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Cc: syzbot+c950cc277150935cc0b5@syzkaller.appspotmail.com, martin.lau@kernel.org, clm@meta.com, ihor.solodrai@linux.dev Date: Tue, 28 Oct 2025 10:18:45 -0700 In-Reply-To: References: <20251028151938.3872003-2-kafai.wan@linux.dev> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2 (3.56.2-1.fc42) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2025-10-28 at 15:45 +0000, bot+bpf-ci@kernel.org wrote: > ``` > commit 16cbc4520eb13d2065d145c76b97fb93035d81e8 > Author: KaFai Wan >=20 > bpf: Fix tnum_overlap to check for zero mask intersection >=20 > This patch adds an early return check in tnum_overlap() to detect > non-overlapping tnums when their masks have no overlapping bits. The > fix addresses a syzbot report where the verifier failed to recognize > that two tnums couldn't represent the same value. >=20 > Reported-by: syzbot+c950cc277150935cc0b5@syzkaller.appspotmail.com >=20 > > diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c > > index f8e70e9c3..1a75b7c9a 100644 > > --- a/kernel/bpf/tnum.c > > +++ b/kernel/bpf/tnum.c > > @@ -163,6 +163,8 @@ bool tnum_overlap(struct tnum a, struct tnum b) > > { > > u64 mu; > >=20 > > + if (a.mask && b.mask && !(a.mask & b.mask)) > > + return false; > ^^^^^ >=20 > Does this logic correctly detect non-overlapping tnums? Consider this > counter-example: >=20 > a =3D (value=3D0x5, mask=3D0xA) // binary: value=3D0101, mask=3D1010 > b =3D (value=3D0xA, mask=3D0x5) // binary: value=3D1010, mask=3D0101 >=20 > The masks are disjoint: (0xA & 0x5) =3D=3D 0, so the new check returns > false (no overlap). >=20 > However, these tnums actually represent overlapping sets: > a represents: {5, 7, 13, 15} // 0b0101, 0b0111, 0b1101, 0b1111 > b represents: {10, 11, 14, 15} // 0b1010, 0b1011, 0b1110, 0b1111 >=20 > Both sets contain 15, so they do overlap. Can this cause incorrect > verifier behavior when is_branch_taken() calls tnum_overlap() to > determine if JEQ/JNE branches are reachable? This is a legit bug, please address. > > mu =3D ~a.mask & ~b.mask; > > return (a.value & mu) =3D=3D (b.value & mu); > > } >=20 >=20 > ``` >=20 > --- > AI reviewed your patch. Please fix the bug or email reply why it's not a = bug. > See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/READM= E.md >=20 > In-Reply-To-Subject: `bpf: Fix tnum_overlap to check for zero mask inters= ection` > CI run summary: https://github.com/kernel-patches/bpf/actions/runs/188801= 08453