Re: [PATCH bpf-next v2 2/3] bpf: Reduce stack frame size by using env->insn_buf for bpf insns

[Yonghong Song <yonghong.song@linux.dev>]

On 7/2/25 1:33 PM, Eduard Zingerman wrote:
> On Wed, 2025-07-02 at 10:11 -0700, Yonghong Song 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>
>> Tested-by: Arnd Bergmann <arnd@arndb.de>
>> Acked-by: Jiri Olsa <jolsa@kernel.org>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
>
>>   kernel/bpf/verifier.c | 189 ++++++++++++++++++++----------------------
>>   1 file changed, 91 insertions(+), 98 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 8b0a25851089..ef53e313d841 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -21010,7 +21010,9 @@ 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];
> Nit: I'd add a comment here, something along the lines:
>       "use env->insn_buf as two independent buffers"

Ack. Sounds a good idea.

>
>>   	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;
> [...]