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([2604:3d08:9880:5900:a18e:a67:fdb6:1a18]) by smtp.gmail.com with ESMTPSA id kh5-20020a170903064500b001e47bf10536sm20927608plb.69.2024.04.29.16.16.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Apr 2024 16:16:18 -0700 (PDT) Message-ID: Subject: Re: [PATCH bpf-next v4 7/7] bpf/verifier: improve code after range computation recent changes. From: Eduard Zingerman To: Cupertino Miranda , bpf@vger.kernel.org Cc: Yonghong Song , Alexei Starovoitov , David Faust , Jose Marchesi , Elena Zannoni , Andrii Nakryiko Date: Mon, 29 Apr 2024 16:16:17 -0700 In-Reply-To: <20240429212250.78420-8-cupertino.miranda@oracle.com> References: <20240429212250.78420-1-cupertino.miranda@oracle.com> <20240429212250.78420-8-cupertino.miranda@oracle.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.44.4-0ubuntu2 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 [...] > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index b6344cead2e2..a6fd10b119ba 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -13695,33 +13695,19 @@ static void scalar_min_max_arsh(struct bpf_reg_= state *dst_reg, > __update_reg_bounds(dst_reg); > } > =20 > -static bool is_const_reg_and_valid(const struct bpf_reg_state *reg, bool= alu32, > - bool *valid) > -{ > - s64 smin_val =3D reg->smin_value; > - s64 smax_val =3D reg->smax_value; > - u64 umin_val =3D reg->umin_value; > - u64 umax_val =3D reg->umax_value; > - s32 s32_min_val =3D reg->s32_min_value; > - s32 s32_max_val =3D reg->s32_max_value; > - u32 u32_min_val =3D reg->u32_min_value; > - u32 u32_max_val =3D reg->u32_max_value; > - bool is_const =3D alu32 ? tnum_subreg_is_const(reg->var_off) : > - tnum_is_const(reg->var_off); > - > +static bool is_valid_const_reg(const struct bpf_reg_state *reg, bool alu= 32) > +{ > if (alu32) { > - if ((is_const && > - (s32_min_val !=3D s32_max_val || u32_min_val !=3D u32_max_val)) |= | > - s32_min_val > s32_max_val || u32_min_val > u32_max_val) > - *valid =3D false; This check first originated in the following commit from 2018: 6f16101e6a8b ("bpf: mark dst unknown on inconsistent {s, u}bounds adjustmen= ts") Back then it was added to handle the following program: 0: (b7) r0 =3D 0 1: (d5) if r0 s<=3D 0x0 goto pc+0 <---- note pc+0 here R0=3Dinv0 R1=3Dctx(id=3D0,off=3D0,imm=3D0) R10=3Dfp0 2: (1f) r0 -=3D r1 R0=3Dinv0 R1=3Dctx(id=3D0,off=3D0,imm=3D0) R10=3Dfp0 verifier internal error: known but bad sbounds Apparently, verifier visited both conditional branches for this program deducing impossible bounds for the 'false' branch. Nowadays is_scalar_branch_taken() should handle such situations w/o issues. Still, I'm not sure if we want to remove this safety check. [...]