From: Yonghong Song <yonghong.song@linux.dev>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Kernel Team <kernel-team@fb.com>,
Martin KaFai Lau <martin.lau@kernel.org>,
Arnd Bergmann <arnd@kernel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: Reduce stack frame size by using env->insn_buf for bpf insns
Date: Wed, 2 Jul 2025 08:29:05 -0700 [thread overview]
Message-ID: <2214fd27-743a-4d50-8ca1-587998b0cc06@linux.dev> (raw)
In-Reply-To: <CAADnVQ+DDozRPgFpFzBZ2NcZJ8WwOUxAY77CSOzkJ8QG=LpaVQ@mail.gmail.com>
On 7/2/25 8:09 AM, Alexei Starovoitov wrote:
> On Tue, Jul 1, 2025 at 10:33 PM Yonghong Song <yonghong.song@linux.dev> wrote:
>> Arnd Bergmann reported an issue ([1]) where clang compiler (less than
>> llvm18) may trigger an error where the stack frame size exceeds the limit.
>> I can reproduce the error like below:
>> kernel/bpf/verifier.c:24491:5: error: stack frame size (2552) exceeds limit (1280) in 'bpf_check'
>> [-Werror,-Wframe-larger-than]
>> kernel/bpf/verifier.c:19921:12: error: stack frame size (1368) exceeds limit (1280) in 'do_check'
>> [-Werror,-Wframe-larger-than]
>>
>> Use env->insn_buf for bpf insns instead of putting these insns on the
>> stack. This can resolve the above 'bpf_check' error. The 'do_check' error
>> will be resolved in the next patch.
>>
>> [1] https://lore.kernel.org/bpf/20250620113846.3950478-1-arnd@kernel.org/
>>
>> Reported-by: Arnd Bergmann <arnd@kernel.org>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>> kernel/bpf/verifier.c | 194 ++++++++++++++++++++----------------------
>> 1 file changed, 91 insertions(+), 103 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 90e688f81a48..29faef51065d 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -20939,26 +20939,27 @@ static bool insn_is_cond_jump(u8 code)
>> static void opt_hard_wire_dead_code_branches(struct bpf_verifier_env *env)
>> {
>> struct bpf_insn_aux_data *aux_data = env->insn_aux_data;
>> - struct bpf_insn ja = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
>> + struct bpf_insn *ja = env->insn_buf;
> This one replaces 8 bytes on stack with an 8 byte pointer.
> Does it actually make a difference in stack usage?
> I have my doubts.
Probably no difference. I just made change here just to remove
*all* 'struct bpf_insn var/var[]' on the stack. But as you suggested,
I will remove this change.
>
>> struct bpf_insn *insn = env->prog->insnsi;
>> const int insn_cnt = env->prog->len;
>> int i;
>>
>> + *ja = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
>> for (i = 0; i < insn_cnt; i++, insn++) {
>> if (!insn_is_cond_jump(insn->code))
>> continue;
>>
>> if (!aux_data[i + 1].seen)
>> - ja.off = insn->off;
>> + ja->off = insn->off;
>> else if (!aux_data[i + 1 + insn->off].seen)
>> - ja.off = 0;
>> + ja->off = 0;
>> else
>> continue;
>>
>> if (bpf_prog_is_offloaded(env->prog->aux))
>> - bpf_prog_offload_replace_insn(env, i, &ja);
>> + bpf_prog_offload_replace_insn(env, i, ja);
>>
>> - memcpy(insn, &ja, sizeof(ja));
>> + memcpy(insn, ja, sizeof(*ja));
>> }
>> }
>>
>> @@ -21017,7 +21018,9 @@ static int opt_remove_nops(struct bpf_verifier_env *env)
>> static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
>> const union bpf_attr *attr)
>> {
>> - struct bpf_insn *patch, zext_patch[2], rnd_hi32_patch[4];
>> + struct bpf_insn *patch;
>> + struct bpf_insn *zext_patch = env->insn_buf;
>> + struct bpf_insn *rnd_hi32_patch = &env->insn_buf[2];
>> struct bpf_insn_aux_data *aux = env->insn_aux_data;
>> int i, patch_len, delta = 0, len = env->prog->len;
>> struct bpf_insn *insns = env->prog->insnsi;
>> @@ -21195,13 +21198,12 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>>
>> if (env->insn_aux_data[i + delta].nospec) {
>> WARN_ON_ONCE(env->insn_aux_data[i + delta].alu_state);
>> - struct bpf_insn patch[] = {
>> - BPF_ST_NOSPEC(),
>> - *insn,
>> - };
>> + struct bpf_insn *patch = &insn_buf[0];
> why &..[0] ? Can it just be patch = insn_buf ?
There are a couple of cases in the existing code, e.g.,
$ grep -C 2 "&insn_buf\[0\]" verifier.c
(BPF_MODE(insn->code) == BPF_PROBE_MEM ||
BPF_MODE(insn->code) == BPF_PROBE_MEMSX)) {
struct bpf_insn *patch = &insn_buf[0];
u64 uaddress_limit = bpf_arch_uaddress_limit();
--
const u8 code_add = BPF_ALU64 | BPF_ADD | BPF_X;
const u8 code_sub = BPF_ALU64 | BPF_SUB | BPF_X;
struct bpf_insn *patch = &insn_buf[0];
bool issrc, isneg, isimm;
u32 off_reg;
I was using '... = &insn_buf[0]' to make code consistent.
Will change to use '... = insn_buf' including the above two cases.
>
>> - cnt = ARRAY_SIZE(patch);
>> - new_prog = bpf_patch_insn_data(env, i + delta, patch, cnt);
>> + *patch++ = BPF_ST_NOSPEC();
>> + *patch++ = *insn;
>> + cnt = patch - insn_buf;
>> + new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
>> if (!new_prog)
>> return -ENOMEM;
>>
>> @@ -21269,13 +21271,12 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>> /* nospec_result is only used to mitigate Spectre v4 and
>> * to limit verification-time for Spectre v1.
>> */
>> - struct bpf_insn patch[] = {
>> - *insn,
>> - BPF_ST_NOSPEC(),
>> - };
>> + struct bpf_insn *patch = &insn_buf[0];
>>
>> - cnt = ARRAY_SIZE(patch);
>> - new_prog = bpf_patch_insn_data(env, i + delta, patch, cnt);
>> + *patch++ = *insn;
>> + *patch++ = BPF_ST_NOSPEC();
>> + cnt = patch - insn_buf;
>> + new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
>> if (!new_prog)
>> return -ENOMEM;
>>
>> @@ -21945,13 +21946,12 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>> u16 stack_depth_extra = 0;
>>
>> if (env->seen_exception && !env->exception_callback_subprog) {
>> - struct bpf_insn patch[] = {
>> - env->prog->insnsi[insn_cnt - 1],
>> - BPF_MOV64_REG(BPF_REG_0, BPF_REG_1),
>> - BPF_EXIT_INSN(),
>> - };
>> + struct bpf_insn *patch = &insn_buf[0];
>>
>> - ret = add_hidden_subprog(env, patch, ARRAY_SIZE(patch));
>> + *patch++ = env->prog->insnsi[insn_cnt - 1];
>> + *patch++ = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
>> + *patch++ = BPF_EXIT_INSN();
>> + ret = add_hidden_subprog(env, insn_buf, patch - insn_buf);
>> if (ret < 0)
>> return ret;
>> prog = env->prog;
>> @@ -21987,20 +21987,18 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>> insn->off == 1 && insn->imm == -1) {
>> bool is64 = BPF_CLASS(insn->code) == BPF_ALU64;
>> bool isdiv = BPF_OP(insn->code) == BPF_DIV;
>> - struct bpf_insn *patchlet;
>> - struct bpf_insn chk_and_sdiv[] = {
>> - BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> - BPF_NEG | BPF_K, insn->dst_reg,
>> - 0, 0, 0),
>> - };
>> - struct bpf_insn chk_and_smod[] = {
>> - BPF_MOV32_IMM(insn->dst_reg, 0),
>> - };
>> + struct bpf_insn *patch = &insn_buf[0];
>> +
>> + if (isdiv)
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> + BPF_NEG | BPF_K, insn->dst_reg,
>> + 0, 0, 0);
>> + else
>> + *patch++ = BPF_MOV32_IMM(insn->dst_reg, 0);
>>
>> - patchlet = isdiv ? chk_and_sdiv : chk_and_smod;
>> - cnt = isdiv ? ARRAY_SIZE(chk_and_sdiv) : ARRAY_SIZE(chk_and_smod);
>> + cnt = patch - insn_buf;
>>
>> - new_prog = bpf_patch_insn_data(env, i + delta, patchlet, cnt);
>> + new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
>> if (!new_prog)
>> return -ENOMEM;
>>
>> @@ -22019,83 +22017,73 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>> bool isdiv = BPF_OP(insn->code) == BPF_DIV;
>> bool is_sdiv = isdiv && insn->off == 1;
>> bool is_smod = !isdiv && insn->off == 1;
>> - struct bpf_insn *patchlet;
>> - struct bpf_insn chk_and_div[] = {
>> - /* [R,W]x div 0 -> 0 */
>> - BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> - BPF_JNE | BPF_K, insn->src_reg,
>> - 0, 2, 0),
>> - BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg),
>> - BPF_JMP_IMM(BPF_JA, 0, 0, 1),
>> - *insn,
>> - };
>> - struct bpf_insn chk_and_mod[] = {
>> - /* [R,W]x mod 0 -> [R,W]x */
>> - BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> - BPF_JEQ | BPF_K, insn->src_reg,
>> - 0, 1 + (is64 ? 0 : 1), 0),
>> - *insn,
>> - BPF_JMP_IMM(BPF_JA, 0, 0, 1),
>> - BPF_MOV32_REG(insn->dst_reg, insn->dst_reg),
>> - };
>> - struct bpf_insn chk_and_sdiv[] = {
>> + struct bpf_insn *patch = &insn_buf[0];
>> +
>> + if (is_sdiv) {
>> /* [R,W]x sdiv 0 -> 0
>> * LLONG_MIN sdiv -1 -> LLONG_MIN
>> * INT_MIN sdiv -1 -> INT_MIN
>> */
>> - BPF_MOV64_REG(BPF_REG_AX, insn->src_reg),
>> - BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> - BPF_ADD | BPF_K, BPF_REG_AX,
>> - 0, 0, 1),
>> - BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> - BPF_JGT | BPF_K, BPF_REG_AX,
>> - 0, 4, 1),
>> - BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> - BPF_JEQ | BPF_K, BPF_REG_AX,
>> - 0, 1, 0),
>> - BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> - BPF_MOV | BPF_K, insn->dst_reg,
>> - 0, 0, 0),
>> + *patch++ = BPF_MOV64_REG(BPF_REG_AX, insn->src_reg);
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> + BPF_ADD | BPF_K, BPF_REG_AX,
>> + 0, 0, 1);
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> + BPF_JGT | BPF_K, BPF_REG_AX,
>> + 0, 4, 1);
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> + BPF_JEQ | BPF_K, BPF_REG_AX,
>> + 0, 1, 0);
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> + BPF_MOV | BPF_K, insn->dst_reg,
>> + 0, 0, 0);
>> /* BPF_NEG(LLONG_MIN) == -LLONG_MIN == LLONG_MIN */
>> - BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> - BPF_NEG | BPF_K, insn->dst_reg,
>> - 0, 0, 0),
>> - BPF_JMP_IMM(BPF_JA, 0, 0, 1),
>> - *insn,
>> - };
>> - struct bpf_insn chk_and_smod[] = {
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> + BPF_NEG | BPF_K, insn->dst_reg,
>> + 0, 0, 0);
>> + *patch++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
>> + *patch++ = *insn;
>> + cnt = patch - insn_buf;
>> + } else if (is_smod) {
>> /* [R,W]x mod 0 -> [R,W]x */
>> /* [R,W]x mod -1 -> 0 */
>> - BPF_MOV64_REG(BPF_REG_AX, insn->src_reg),
>> - BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> - BPF_ADD | BPF_K, BPF_REG_AX,
>> - 0, 0, 1),
>> - BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> - BPF_JGT | BPF_K, BPF_REG_AX,
>> - 0, 3, 1),
>> - BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> - BPF_JEQ | BPF_K, BPF_REG_AX,
>> - 0, 3 + (is64 ? 0 : 1), 1),
>> - BPF_MOV32_IMM(insn->dst_reg, 0),
>> - BPF_JMP_IMM(BPF_JA, 0, 0, 1),
>> - *insn,
>> - BPF_JMP_IMM(BPF_JA, 0, 0, 1),
>> - BPF_MOV32_REG(insn->dst_reg, insn->dst_reg),
>> - };
>> -
>> - if (is_sdiv) {
>> - patchlet = chk_and_sdiv;
>> - cnt = ARRAY_SIZE(chk_and_sdiv);
>> - } else if (is_smod) {
>> - patchlet = chk_and_smod;
>> - cnt = ARRAY_SIZE(chk_and_smod) - (is64 ? 2 : 0);
>> + *patch++ = BPF_MOV64_REG(BPF_REG_AX, insn->src_reg);
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_ALU64 : BPF_ALU) |
>> + BPF_ADD | BPF_K, BPF_REG_AX,
>> + 0, 0, 1);
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> + BPF_JGT | BPF_K, BPF_REG_AX,
>> + 0, 3, 1);
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> + BPF_JEQ | BPF_K, BPF_REG_AX,
>> + 0, 3 + (is64 ? 0 : 1), 1);
>> + *patch++ = BPF_MOV32_IMM(insn->dst_reg, 0);
>> + *patch++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
>> + *patch++ = *insn;
>> + *patch++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
>> + *patch++ = BPF_MOV32_REG(insn->dst_reg, insn->dst_reg);
>> + cnt = (patch - insn_buf) - (is64 ? 2 : 0);
>> + } else if (isdiv) {
>> + /* [R,W]x div 0 -> 0 */
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> + BPF_JNE | BPF_K, insn->src_reg,
>> + 0, 2, 0);
>> + *patch++ = BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg);
>> + *patch++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
>> + *patch++ = *insn;
>> + cnt = patch - insn_buf;
>> } else {
>> - patchlet = isdiv ? chk_and_div : chk_and_mod;
>> - cnt = isdiv ? ARRAY_SIZE(chk_and_div) :
>> - ARRAY_SIZE(chk_and_mod) - (is64 ? 2 : 0);
>> + /* [R,W]x mod 0 -> [R,W]x */
>> + *patch++ = BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
>> + BPF_JEQ | BPF_K, insn->src_reg,
>> + 0, 1 + (is64 ? 0 : 1), 0);
>> + *patch++ = *insn;
>> + *patch++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
>> + *patch++ = BPF_MOV32_REG(insn->dst_reg, insn->dst_reg);
>> + cnt = (patch - insn_buf) - (is64 ? 2 : 0);
> hmm. why populate two extra insn just to drop them ?
> Don't add the last two insn instead.
> I would also simplify the previous jmp32 vs jmp generation by
> testing if (is64) once and at the end cnt = patch - insn_buf;
Make sense. Will do.
>
> --
> pw-bot: cr
>
prev parent reply other threads:[~2025-07-02 15:29 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-07-02 5:33 [PATCH bpf-next 1/2] bpf: Reduce stack frame size by using env->insn_buf for bpf insns Yonghong Song
2025-07-02 5:33 ` [PATCH bpf-next 2/2] bpf: Avoid putting struct bpf_scc_callchain variables on the stack Yonghong Song
2025-07-02 8:49 ` Jiri Olsa
2025-07-02 8:54 ` [PATCH bpf-next 1/2] bpf: Reduce stack frame size by using env->insn_buf for bpf insns Jiri Olsa
2025-07-02 9:28 ` Arnd Bergmann
2025-07-02 15:09 ` Alexei Starovoitov
2025-07-02 15:29 ` Yonghong Song [this message]
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