From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from 66-220-144-179.mail-mxout.facebook.com (66-220-144-179.mail-mxout.facebook.com [66.220.144.179]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7694038236C for ; Sat, 8 Aug 2026 19:04:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=66.220.144.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786215841; cv=none; b=A6qb8whjNBRdTqttCrJ1SZQ6AJSCbNeYcaqGsquAxyYhydmzmZ62XN8C+kbULT0uo93CM63ED4M0S7hOjXDeLZhNLgOQWvE98/pdIae9tgnC3CNtwFhddHnhQBg5yK2KMajPlmZX3NJeuPTP3W8RWb7M+GCt+6y//wXU29zSsMo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786215841; c=relaxed/simple; bh=EPOS70lLY6JdjRT5r/QE03eQx710dJDiUZPWHLo8n/4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=klRtwtNGYClxUh9i1PcsedAmpsyDyIivxsi3B3kKHOS7rbfSAjnlpx900lSQrRnDsp6Kc16ah44ga6JUhQufnFIaqOSw77KlCtoWzwB4spMcV33KeUJ0WvAuH4asf9uGlwIUY+mV/nY9vyVpcJpkabzJ2wfkDDJN071gdUUTrNc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev; spf=fail smtp.mailfrom=linux.dev; arc=none smtp.client-ip=66.220.144.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=linux.dev Received: by devvm16039.vll0.facebook.com (Postfix, from userid 128203) id E7FEE22CA31C9F; Sat, 8 Aug 2026 12:03:47 -0700 (PDT) From: Yonghong Song To: bpf@vger.kernel.org Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Eduard Zingerman , kernel-team@fb.com Subject: [PATCH bpf-next v3 05/13] bpf: Account R2 of register-pair returns in live register analysis Date: Sat, 8 Aug 2026 12:03:47 -0700 Message-ID: <20260808190347.1898998-1-yonghong.song@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260808190322.1896580-1-yonghong.song@linux.dev> References: <20260808190322.1896580-1-yonghong.song@linux.dev> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable 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 a callee's R2 could be considered dead and cleaned from checkpointed states, which would allow unsound state pruning= . Mark R2 as read at the BPF_EXIT of a subprogram that does return a regist= er pair. bpf_compute_live_registers() walks the instructions in order and env->subprog_info[] is sorted by subprogram start, so the containing subprogram is tracked with a running index and its return convention is queried once per subprogram through bpf_ret_reg_pair(). Marking R2 at every exit instead would be simpler, but R2 would then stay live backwards across any call that is not followed by a write to R2, whi= ch is nearly every program, and would needlessly hurt state pruning. Signed-off-by: Yonghong Song --- kernel/bpf/liveness.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) diff --git a/kernel/bpf/liveness.c b/kernel/bpf/liveness.c index ef9a5a922887..451edd74fa6f 100644 --- a/kernel/bpf/liveness.c +++ b/kernel/bpf/liveness.c @@ -2062,10 +2062,15 @@ static inline u32 mask_widen(u32 m) { return m | = (m << 16); } static inline u16 mask_lo(u32 m) { return (u16)m; } static inline u16 mask_hi(u32 m) { return (u16)(m >> 16); } =20 -/* Compute info->{use,def} fields for the instruction */ +/* + * Compute info->{use,def} fields for the instruction. @ret_reg_pair tel= ls + * whether the subprogram containing @insn returns its value in the R0:R= 2 + * register pair, which matters for BPF_EXIT. + */ static void compute_insn_live_regs(struct bpf_verifier_env *env, struct bpf_insn *insn, - struct insn_live_regs *info) + struct insn_live_regs *info, + bool ret_reg_pair) { struct bpf_call_summary cs; const u8 class =3D BPF_CLASS(insn->code); @@ -2196,7 +2201,7 @@ static void compute_insn_live_regs(struct bpf_verif= ier_env *env, break; case BPF_EXIT: def =3D 0; - use =3D r0; + use =3D ret_reg_pair ? (r0 | reg64_mask(BPF_REG_2)) : r0; break; case BPF_CALL: def =3D ALL_CALLER_SAVED_REGS; @@ -2233,8 +2238,8 @@ int bpf_compute_live_registers(struct bpf_verifier_= env *env) struct insn_live_regs *state; int insn_cnt =3D env->prog->len; u64 pos, insn_pos; - int err =3D 0, i, j; - bool changed; + int err =3D 0, i, j, subprog, start, end; + bool changed, ret_reg_pair; =20 /* Use the following algorithm: * - define the following: @@ -2261,8 +2266,14 @@ int bpf_compute_live_registers(struct bpf_verifier= _env *env) goto out; } =20 - for (i =3D 0; i < insn_cnt; ++i) - compute_insn_live_regs(env, &insns[i], &state[i]); + for (subprog =3D 0; subprog < env->subprog_cnt; subprog++) { + start =3D env->subprog_info[subprog].start; + end =3D env->subprog_info[subprog + 1].start; + ret_reg_pair =3D bpf_ret_reg_pair(env, subprog); + + for (i =3D start; i < end; ++i) + compute_insn_live_regs(env, &insns[i], &state[i], ret_reg_pair); + } =20 /* Forward pass: resolve stack access through FP-derived pointers */ err =3D bpf_compute_subprog_arg_access(env); --=20 2.53.0-Meta