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([2620:10d:c090:500::2:634c]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-2b7102fd2efsm757052eec.12.2026.01.20.13.54.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Jan 2026 13:54:16 -0800 (PST) Message-ID: Subject: Re: [PATCH bpf-next v5 1/2] bpf: Add range tracking for BPF_DIV and BPF_MOD From: Eduard Zingerman To: Alexei Starovoitov Cc: Yazhou Tang , bpf , Alexei Starovoitov , Daniel Borkmann , John Fastabend , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Yazhou Tang , Shenghao Yuan , Tianci Cao , syzbot@syzkaller.appspotmail.com Date: Tue, 20 Jan 2026 13:54:13 -0800 In-Reply-To: References: <20260119085458.182221-1-tangyazhou@zju.edu.cn> <20260119085458.182221-2-tangyazhou@zju.edu.cn> <57dbe0b62e2cf0370f90825f9508c466cd673704.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.2 (3.58.2-1.fc43) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2026-01-20 at 13:03 -0800, Alexei Starovoitov wrote: > On Tue, Jan 20, 2026 at 10:51=E2=80=AFAM Eduard Zingerman wrote: > >=20 > > On Mon, 2026-01-19 at 16:54 +0800, Yazhou Tang wrote: > >=20 > > [...] > >=20 > > Hi Yazhou, Alexei, > >=20 > > Sorry for chiming in late in the series. > >=20 > > > diff --git a/tools/testing/selftests/bpf/progs/verifier_value_illegal= _alu.c b/tools/testing/selftests/bpf/progs/verifier_value_illegal_alu.c > > > index 2129e4353fd9..4d8273c258d5 100644 > > > --- a/tools/testing/selftests/bpf/progs/verifier_value_illegal_alu.c > > > +++ b/tools/testing/selftests/bpf/progs/verifier_value_illegal_alu.c > > > @@ -173,14 +173,15 @@ __naked void flow_keys_illegal_variable_offset_= alu(void) > > > asm volatile(" \ > > > r6 =3D r1; \ > > > r7 =3D *(u64*)(r6 + %[flow_keys_off]); \ > > > - r8 =3D 8; \ > > > - r8 /=3D 1; \ > > > + call %[bpf_get_prandom_u32]; \ > > > + r8 =3D r0; \ > >=20 > > I suggest we special case `rX /=3D 1`. > > Maybe in the is_safe_to_compute_dst_reg_range(). > > This is an old trick that might be used in the wild. > > And optimizer should remove any such instructions when compiling from C= . > > So I don't think adding special case here would hinder verifier versati= lity. >=20 > and to standardize now and forever that /=3D1 makes it unbounded? > I'd rather not. JITs don't special case the divisor 1 and in runtime > the /=3D1 trick is quite costly. Integer division is the slowest instruct= ion > on any CPU architecture. > We have few selftests that adopted that trick, but let's not continue > and not encourage it going forward. > Let's figure out a different way to tell the verifier that the scalar > is unbounded. Fair enough. > I suspect ^=3D0; ^=3D0; will destroy the bounds just as well > and will be fast to execute. scalar_min_max_xor() tries to compute min and max, does not nuke register state. Andrii suggested adding a kfunc or instruction for this purpose some time ago. Anyway, that's unrelated if we don't want to keep /=3D 1.