From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from 66-220-155-179.mail-mxout.facebook.com (66-220-155-179.mail-mxout.facebook.com [66.220.155.179]) (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 6D7DD3890EE for ; Sat, 8 Aug 2026 19:04:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=66.220.155.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786215847; cv=none; b=sZ841aLiAccEFhmNBVsbuXC4DQwzlEsP9IGPc1HWK5yvs8zdwnt1NXUdq2zMUMhA2lvySkMth3lDZWBOzJ0xGK7dmuiHLpM2TG3yEDMYOKYjFJw0cpDVcXYwFfpdjM2kRU3hDxS2zNOqSzEHWaTz+3paP91nJu4FrKGw1tXvCI8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786215847; c=relaxed/simple; bh=3NNcSTSJufgqBmD2iOVcsDgKhCRpQJ+RtQ8fJkrLGeI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=WzZAYESV5n/cS7SIiGCyxptskRPAc+MTk+Ap9apBX4kQhC2ID1eT9AjG4b59YD7jC1cR3FS9EyKeTQsLk9aDHehpoJWLmLGWzEZRMdSBeNLRpapE4OOKQKKWAdWQlGYc6FSH7jQCIZ6VcsOiOf0wbIjmQCDihMq7butGIp7035M= 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=66.220.155.179 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 2E56F22CA31D03; Sat, 8 Aug 2026 12:03:58 -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 07/13] bpf: Add verifier support for 16-byte returns in R0:R2 Date: Sat, 8 Aug 2026 12:03:58 -0700 Message-ID: <20260808190358.1899615-1-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 LLVM 23 added support for returning a value in two registers for an __int128, or a struct/union whose size is greater than 8 but not more tha= n 16 bytes. See LLVM patches [1] and [2]. Before LLVM 23 the BPF backend could not return these values at all. A by-value struct or union return (of any size) was rejected at compile tim= e with: error: aggregate returns are not supported and an __int128 return failed later in the backend with: fatal error: error in backend: unable to allocate function return #1 Both are resolved in LLVM 23, which lowers such returns into the R0:R2 register pair. This patch adds handling for returns greater than 8 bytes in several places: BPF subprogram returns (the main program, and both global and static subprograms) and kfunc returns. The R0:R2 convention is only implemented in the JIT. The BPF interpreter has no notion of a second return register: a BPF-to-BPF call goes through JMP_CALL_ARGS and a BPF_EXIT hands back BPF_R0 alone, so a caller reading R2 would see a stale value. Force the JIT wherever a caller can observe t= he pair, that is at the call to a global subprogram in check_func_call() and at the return from a static subprogram in prepare_func_exit(). Kfunc call= s need no separate handling since bpf_add_kfunc_call() already sets jit_required for every kfunc call. A by-value struct or union returned by a kfunc must be composed only of scalars, since the verifier models the returned register bits as an unkno= wn scalar and a pointer field would otherwise be laundered into one, escapin= g provenance and reference tracking. A global subprogram must return a scalar in every return register. The existing exemption for arena pointers now applies only when the return value fits in R0 alone: both halves of a register pair carry a piece of a >8 byte scalar, so an arena pointer in either of them is a leak rather th= an a legitimate return value. A subprogram whose whole return value is an arena pointer is unaffected. A static subprogram is handled differently. The verifier walks into its frame, so prepare_func_exit() propagates the return register(s) to the caller. R0 holding a stack pointer has long been rejected outright there, but R2 is deliberately not treated the same way. LLVM owns both sides of = a static call and is not bound by the ABI, so even with a 9..16 byte declar= ed return type it may leave R2 untouched when the caller only consumes the l= ow half; R2 can then hold an incidental stack pointer that is not a return value at all, and rejecting the program would be a false positive. Propagating the register as is would be worse: the callee frame is freed immediately afterwards, leaving the caller with a PTR_TO_STACK that refer= s to a frame which no longer exists. So the caller's R2 is marked uninitialized instead, and only a caller that actually reads the returned upper half fails. As with R0, a pointer into the caller's own frame is scrubbed too, which is conservative but keeps the two registers consisten= t. Once callers read R0:R2, an extension program can no longer replace a function with a >8 byte return value: an extension's own return is capped at 8 bytes by the program-exit check above, so it would leave R2 stale for the target's callers. btf_check_type_match() cannot catch this, as it compares return types by btf_type->info only and an int carries no vlen, so a 16-byte __int128 and an 8-byte long compare equal. Reject such an attach in bpf_check_attach_target() instead. [1] https://github.com/llvm/llvm-project/pull/190894 [2] https://github.com/llvm/llvm-project/pull/206876 Signed-off-by: Yonghong Song --- kernel/bpf/verifier.c | 119 ++++++++++++++++++++++++++++++++++++++---- 1 file changed, 109 insertions(+), 10 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 0c070957a52b..41c47bcc3b0a 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -419,6 +419,9 @@ static u32 ret_regs_cnt(u32 size) return size > 8 && size <=3D 16 ? 2 : 1; } =20 +/* Registers holding a function return value, in order. See ret_regs_cnt= (). */ +static const int ret_regs[] =3D { BPF_REG_0, BPF_REG_2 }; + /* * Resolve the return convention of every subprogram once, so that * bpf_ret_reg_pair() is a plain flag test on the hot paths that use it. @@ -9444,6 +9447,7 @@ static int check_func_call(struct bpf_verifier_env = *env, struct bpf_insn *insn, u16 callee_incoming, stack_arg_cnt; struct bpf_func_state *caller; int err, subprog, target_insn; + u32 i, nregs; =20 target_insn =3D *insn_idx + insn->imm + 1; subprog =3D bpf_find_subprog(env, target_insn); @@ -9486,9 +9490,22 @@ static int check_func_call(struct bpf_verifier_env= *env, struct bpf_insn *insn, clear_caller_saved_regs(env, caller->regs); invalidate_outgoing_stack_args(env, cur_func(env)); =20 - /* All non-void global functions return a 64-bit SCALAR_VALUE. */ + /* + * A non-void global function returns a 64-bit SCALAR_VALUE in + * R0, or a >8 byte SCALAR_VALUE in the R0:R2 register pair. + */ if (!subprog_returns_void(env, subprog)) { - mark_reg_unknown(env, caller->regs, BPF_REG_0); + nregs =3D bpf_ret_reg_pair(env, subprog) ? 2 : 1; + /* + * The R0:R2 return convention is only implemented in the + * JIT: the interpreter propagates BPF_R0 alone out of a + * subprogram, so a caller reading R2 would see a stale + * value. Force the JIT once a caller can observe the pair. + */ + if (nregs > 1) + env->prog->jit_required =3D 1; + for (i =3D 0; i < nregs; i++) + mark_reg_unknown(env, caller->regs, ret_regs[i]); } =20 if (env->subprog_info[subprog].might_throw) { @@ -9810,10 +9827,14 @@ static int prepare_func_exit(struct bpf_verifier_= env *env, int *insn_idx) struct bpf_func_state *caller, *callee; struct bpf_reg_state *r0; bool in_callback_fn; + u32 i, nregs; int err; =20 callee =3D state->frame[state->curframe]; r0 =3D &callee->regs[BPF_REG_0]; + nregs =3D bpf_ret_reg_pair(env, callee->subprogno) ? 2 : 1; + if (nregs > 1) + env->prog->jit_required =3D 1; if (r0->type =3D=3D PTR_TO_STACK) { /* technically it's ok to return caller's stack pointer * (or caller's caller's pointer) back to the caller, @@ -9849,8 +9870,21 @@ static int prepare_func_exit(struct bpf_verifier_e= nv *env, int *insn_idx) return -EFAULT; } } else { - /* return to the caller whatever r0 had in the callee */ - caller->regs[BPF_REG_0] =3D *r0; + /* return to the caller whatever the callee had in the + * return register(s) + */ + for (i =3D 0; i < nregs; i++) + caller->regs[ret_regs[i]] =3D callee->regs[ret_regs[i]]; + + /* R2 carries only the upper half of a register pair return + * value. A stack pointer must not escape the callee (see the + * R0 case above), but there is no need to reject the whole + * program for it: hand the caller an uninitialized R2 instead, + * so that only a caller actually using the returned pointer + * fails. + */ + if (nregs > 1 && caller->regs[BPF_REG_2].type =3D=3D PTR_TO_STACK) + bpf_mark_reg_not_init(env, &caller->regs[BPF_REG_2]); } =20 /* for callbacks like bpf_loop or bpf_for_each_map_elem go back to call= site, @@ -10747,6 +10781,18 @@ static int check_helper_call(struct bpf_verifier= _env *env, struct bpf_insn *insn return 0; } =20 +/* Mark the register(s) holding a @size byte kfunc return value as unkno= wn + * scalars. Both halves of a register pair are treated the same way. + */ +static void mark_kfunc_ret_regs(struct bpf_verifier_env *env, + struct bpf_reg_state *regs, u32 size) +{ + u32 i, nregs =3D ret_regs_cnt(size); + + for (i =3D 0; i < nregs; i++) + mark_reg_unknown(env, regs, ret_regs[i]); +} + static bool is_kfunc_acquire(struct bpf_call_arg_meta *meta) { return meta->kfunc_flags & KF_ACQUIRE; @@ -13223,10 +13269,25 @@ static int check_kfunc_call(struct bpf_verifier= _env *env, struct bpf_insn *insn, } =20 if (btf_type_is_scalar(t)) { - mark_reg_unknown(env, regs, BPF_REG_0); + mark_kfunc_ret_regs(env, regs, t->size); if (meta.btf =3D=3D btf_vmlinux && (meta.func_id =3D=3D special_kfunc_= list[KF_bpf_res_spin_lock] || meta.func_id =3D=3D special_kfunc_list[KF_bpf_res_spin_lock_irqsav= e])) __mark_reg_const_zero(env, ®s[BPF_REG_0]); + } else if (btf_type_is_struct(t)) { + /* + * The returned struct comes back as raw register bits modeled + * as an unknown scalar, so it must contain only scalars: + * otherwise a pointer field would be laundered into a scalar + * and escape provenance and reference tracking. + */ + if (!__btf_type_is_scalar_struct(env, desc_btf, t, 0)) { + verbose(env, + "kernel function %s returns %s %s that is not composed of scalars\n"= , + func_name, btf_type_str(t), + btf_name_by_offset(desc_btf, t->name_off)); + return -EINVAL; + } + mark_kfunc_ret_regs(env, regs, t->size); } else if (btf_type_is_ptr(t)) { ptr_type =3D btf_type_skip_modifiers(desc_btf, t->type, &ptr_type_id); err =3D check_special_kfunc(env, &meta, regs, insn_aux, ptr_type, desc= _btf); @@ -16678,7 +16739,8 @@ static int check_return_code(struct bpf_verifier_= env *env, int regno, const char return 0; } =20 -static int check_global_ret_scalar_reg(struct bpf_verifier_env *env, u32= regno) +static int check_global_ret_scalar_reg(struct bpf_verifier_env *env, u32= regno, + bool allow_arena_ptr_return) { struct bpf_reg_state *reg; int err; @@ -16688,7 +16750,7 @@ static int check_global_ret_scalar_reg(struct bpf= _verifier_env *env, u32 regno) return err; =20 /* Pointers to arena are safe to pass between subprograms. */ - if (is_arena_reg(env, regno)) + if (allow_arena_ptr_return && is_arena_reg(env, regno)) return 0; =20 if (is_pointer_value(env, regno)) { @@ -16710,11 +16772,26 @@ static int check_global_subprog_return_code(str= uct bpf_verifier_env *env) { struct bpf_func_state *cur_frame =3D cur_func(env); u32 subprog =3D cur_frame->subprogno; + u32 i, nregs; + int err; =20 if (subprog_returns_void(env, subprog)) return 0; =20 - return check_global_ret_scalar_reg(env, BPF_REG_0); + /* + * An arena pointer is only a legitimate return value when it is the + * whole of it, that is when it is returned in R0 alone. Both halves of + * a register pair carry a piece of a >8 byte scalar, so an arena + * pointer in either of them is a leak. + */ + nregs =3D bpf_ret_reg_pair(env, subprog) ? 2 : 1; + for (i =3D 0; i < nregs; i++) { + err =3D check_global_ret_scalar_reg(env, ret_regs[i], nregs =3D=3D 1); + if (err) + return err; + } + + return 0; } =20 /* Bitmask with 1s for all caller saved registers */ @@ -17203,10 +17280,16 @@ static int process_bpf_exit_full(struct bpf_ver= ifier_env *env, */ if (cur_frame->subprogno && !cur_frame->in_async_callback_fn && - !cur_frame->in_exception_callback_fn) + !cur_frame->in_exception_callback_fn) { err =3D check_global_subprog_return_code(env); - else + } else { + if (!cur_frame->subprogno && bpf_ret_reg_pair(env, 0)) { + verbose(env, + "return value larger than 8 bytes is not supported at program exit\n= "); + return -EINVAL; + } err =3D check_return_code(env, BPF_REG_0, "R0"); + } if (err) return err; return PROCESS_BPF_EXIT; @@ -19366,6 +19449,22 @@ int bpf_check_attach_target(struct bpf_verifier_= log *log, return -EOPNOTSUPP; } =20 + /* + * An extension replaces the target outright, so it has to match + * the target's return convention. Its own return value is capped + * at 8 bytes (a >8 byte program return is rejected at BPF_EXIT), + * so it can never fill the R0:R2 pair the target's callers read. + * This cannot be left to btf_check_type_match() above, which + * compares return types by btf_type->info only: an int carries no + * vlen, so a 16-byte __int128 and an 8-byte long compare equal. + */ + if (prog_extension && tgt_info->fmodel.ret_size > 8) { + bpf_log(log, + "Cannot replace function %s with a >8 byte return value\n", + tname); + return -EOPNOTSUPP; + } + /* * *.multi programs don't need an address during program * verification, we just take the module ref if needed. --=20 2.53.0-Meta