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Thu, 09 Jul 2026 14:07:46 -0700 (PDT) Message-ID: <825b731a390d0ecb7827c5a421aebd1dd1ee8d84.camel@gmail.com> Subject: Re: [PATCH bpf-next 05/12] bpf: Account R2 of register-pair returns in live register analysis From: Eduard Zingerman To: Yonghong Song , bpf@vger.kernel.org Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , kernel-team@fb.com Date: Thu, 09 Jul 2026 14:07:43 -0700 In-Reply-To: <20260708201005.2158063-1-yonghong.song@linux.dev> References: <20260708200939.2153664-1-yonghong.song@linux.dev> <20260708201005.2158063-1-yonghong.song@linux.dev> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2-10 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Wed, 2026-07-08 at 13:10 -0700, Yonghong Song wrote: > A BPF_EXIT of a subprogram returning a value larger than 8 bytes (a > struct/union or an __int128) reads R2 as well as R0, since the second hal= f > of the return value is passed back in R2. compute_insn_live_regs() only > marked R0 used at exit, so the callee's R2 could be considered dead and > cleaned from checkpointed states, which would allow unsound state pruning= . >=20 > Mark R2 used at a BPF_EXIT when the enclosing subprogram returns via the > R0:R2 register pair. The call site already treats the caller-saved > registers R0-R5 as clobbered, so R2 as a result register is covered there= . >=20 > For example, for a subprogram returning an __int128, the live-registers > dump (log level 2) shows both R0 and R2 live before the callee's exit > (insn 5): >=20 > =C2=A0 Live regs before insn: > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 0: .12345.... (85) call pc+2 > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 1: ..2....... (bf) r0 =3D r2 > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 2: 0......... (95) exit > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 3: .......... (b7) r0 =3D 1 > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 4: 0......... (b7) r2 =3D 2 > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 5: 0.2....... (95) exit >=20 > Signed-off-by: Yonghong Song > --- > =C2=A0kernel/bpf/liveness.c | 13 ++++++++++--- > =C2=A01 file changed, 10 insertions(+), 3 deletions(-) >=20 > diff --git a/kernel/bpf/liveness.c b/kernel/bpf/liveness.c > index 0aadfbae0acc..0977d32fcf01 100644 > --- a/kernel/bpf/liveness.c > +++ b/kernel/bpf/liveness.c > @@ -2062,7 +2062,8 @@ struct insn_live_regs { > =C2=A0/* Compute info->{use,def} fields for the instruction */ > =C2=A0static void compute_insn_live_regs(struct bpf_verifier_env *env, > =C2=A0 =C2=A0=C2=A0 struct bpf_insn *insn, > - =C2=A0=C2=A0 struct insn_live_regs *info) > + =C2=A0=C2=A0 struct insn_live_regs *info, > + =C2=A0=C2=A0 int subprog) > =C2=A0{ > =C2=A0 struct bpf_call_summary cs; > =C2=A0 u8 class =3D BPF_CLASS(insn->code); > @@ -2175,6 +2176,8 @@ static void compute_insn_live_regs(struct bpf_verif= ier_env *env, > =C2=A0 case BPF_EXIT: > =C2=A0 def =3D 0; > =C2=A0 use =3D r0; > + if (bpf_ret_reg_pair(env, subprog)) > + use |=3D BIT(BPF_REG_2); > =C2=A0 break; > =C2=A0 case BPF_CALL: > =C2=A0 def =3D ALL_CALLER_SAVED_REGS; > @@ -2237,8 +2240,12 @@ int bpf_compute_live_registers(struct bpf_verifier= _env *env) > =C2=A0 goto out; > =C2=A0 } > =C2=A0 > - for (i =3D 0; i < insn_cnt; ++i) > - compute_insn_live_regs(env, &insns[i], &state[i]); > + for (i =3D 0, j =3D 0; i < insn_cnt; ++i) { > + /* Advance to the subprog that contains instruction i. */ > + while (j + 1 < env->subprog_cnt && env->subprog_info[j + 1].start <=3D= i) > + j++; This loop is very in-elegant. The outer loop can be changed to iterate subprograms, with inner loop iterating instructions. Or just use bpf_find_subprog() when processing EXIT. > + compute_insn_live_regs(env, &insns[i], &state[i], j); > + } > =C2=A0 > =C2=A0 /* Forward pass: resolve stack access through FP-derived pointers = */ > =C2=A0 err =3D bpf_compute_subprog_arg_access(env);