Re: [PATCH bpf-next v2 3/3] bpf: Avoid putting struct bpf_scc_callchain variables on the stack

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

On 7/2/25 12:03 PM, Eduard Zingerman wrote:
> On Wed, 2025-07-02 at 10:11 -0700, Yonghong Song wrote:
>> Add a 'struct bpf_scc_callchain callchain' field in bpf_verifier_env.
>> This way, the previous bpf_scc_callchain local variables can be
>> replaced by taking address of env->callchain. This can reduce stack
>> usage and fix the following error:
>>      kernel/bpf/verifier.c:19921:12: error: stack frame size (1368) exceeds limit (1280) in 'do_check'
>>          [-Werror,-Wframe-larger-than]
>>
>> Reported-by: Arnd Bergmann <arnd@kernel.org>
>> Acked-by: Jiri Olsa <jolsa@kernel.org>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
> Oh, well.
> I liked stack allocation for callchain object, because it emphasized
> its ephemeral by-value status.
>
> The changes lgtm, all places with callchain stack allocation replaced.
>
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
>
> [...]
>
>>   include/linux/bpf_verifier.h |  1 +
>>   kernel/bpf/verifier.c        | 36 ++++++++++++++++++------------------
>>   2 files changed, 19 insertions(+), 18 deletions(-)
>>
>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>> index 7e459e839f8b..e2c175d608bb 100644
>> --- a/include/linux/bpf_verifier.h
>> +++ b/include/linux/bpf_verifier.h
>> @@ -841,6 +841,7 @@ struct bpf_verifier_env {
>>   	char tmp_str_buf[TMP_STR_BUF_LEN];
>>   	struct bpf_insn insn_buf[INSN_BUF_SIZE];
>>   	struct bpf_insn epilogue_buf[INSN_BUF_SIZE];
>> +	struct bpf_scc_callchain callchain;
> Nit: maybe a comment here about this being a scratch buffer?

Maybe I can just change the variable name from 'callchain'
to 'callchain_buf' so it will be clear it is a scratch buffer?

>
>>   	/* array of pointers to bpf_scc_info indexed by SCC id */
>>   	struct bpf_scc_info **scc_info;
>>   	u32 scc_cnt;
> [...]