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([2620:10d:c090:500::6:84a4]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-320d61e1568sm8727785eec.11.2026.08.14.16.37.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 14 Aug 2026 16:37:03 -0700 (PDT) Message-ID: <206e8cd41f7955e6526b78df4f9bfd2307b13260.camel@gmail.com> Subject: Re: [PATCH bpf-next v5 04/11] bpf: Track R2 of register-pair returns in precision backtracking From: Eduard Zingerman To: Yonghong Song , bpf@vger.kernel.org Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , kernel-team@fb.com Date: Fri, 14 Aug 2026 16:37:02 -0700 In-Reply-To: <20260813200230.1995389-1-yonghong.song@linux.dev> References: <20260813200210.1991507-1-yonghong.song@linux.dev> <20260813200230.1995389-1-yonghong.song@linux.dev> Content-Type: multipart/mixed; boundary="=-MzDcU1QfuFLPQ4rwrxF8" User-Agent: Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 --=-MzDcU1QfuFLPQ4rwrxF8 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-08-13 at 13:02 -0700, Yonghong Song wrote: ... > @@ -520,7 +541,41 @@ static int backtrack_insn(struct bpf_verifier_env *e= nv, int idx, int subseq_idx, > return -EFAULT; > } > } else if (opcode =3D=3D BPF_EXIT) { > - bool r0_precise; > + bool from_subprog_call, r0_precise, r2_precise; > + struct bpf_insn *call; > + int subprog; > + > + /* BPF_EXIT in subprog or callback always returns > + * right after the call instruction, so by checking > + * whether the instruction at subseq_idx-1 is subprog > + * call or not we can distinguish actual exit from > + * *subprog* from exit from *callback*. In the former > + * case, we need to propagate the precision of the > + * return registers, if necessary. In the latter we > + * never do that. > + */ > + from_subprog_call =3D subseq_idx - 1 >=3D 0 && > + bpf_pseudo_call(&env->prog->insnsi[subseq_idx - 1]); > + > + /* Sample the return registers before the callback > + * handling below clears R1-R5: unlike R0, R2 is an > + * argument register as well, so that clear would drop > + * a pair return on the floor. > + */ > + r0_precise =3D from_subprog_call && bt_is_reg_set(bt, BPF_REG_0); > + r2_precise =3D false; > + if (from_subprog_call && bt_is_reg_set(bt, BPF_REG_2)) { > + call =3D &env->prog->insnsi[subseq_idx - 1]; > + subprog =3D bpf_find_subprog(env, subseq_idx + call->imm); > + if (subprog < 0) > + return -EFAULT; > + /* Only a callee that does return a pair defines > + * R2. Leave the mask alone otherwise, so that > + * the check below still catches an R2 that has > + * no business being set. > + */ > + r2_precise =3D bpf_ret_reg_pair(env, subprog); > + } > =20 > /* Backtracking to a nested function call, 'idx' is a part of > * the inner frame 'subseq_idx' is a part of the outer frame. I still think that the above complications are unnecessary. The patch could be simplified by assuming that R2 always propagates w/o loosing verification safety. 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