Re: [PATCH bpf-next v1 1/3] bpf: Show more useful info in stack depth stats

On Sun Aug 2, 2026 at 11:02 PM CEST, Eduard Zingerman wrote:
> On Sun, 2026-08-02 at 01:03 +0200, Kumar Kartikeya Dwivedi wrote:
>> Currently, the output of stack depth statistics is two crude, with a
>> list of captured stack depths ordered by the subprog numbers. The actual
>> subprog numbers are determined by libbpf, hence it is hard to associate
>> the stack depth statistic back to the subprog by name.
>>
>> Change the format to:
>> stack depth <subprog>=<depth> ... max=<depth>
>>
>> In case the subprog name is not specified, use subprog[N] as the string.
>>
>> Suggested-by: Andrii Nakryiko <[email protected]>
>> Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
>> ---
>>  kernel/bpf/verifier.c                         | 16 ++++++++++++----
>>  .../bpf/progs/verifier_basic_stack.c          |  4 ++--
>>  .../bpf/progs/verifier_bpf_fastcall.c         | 19 +++++++++++--------
>>  .../bpf/progs/verifier_private_stack.c        |  9 ++++++---
>>  .../selftests/bpf/progs/verifier_var_off.c    |  4 ++--
>>  5 files changed, 33 insertions(+), 19 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 8d0635ee48c7..51ac0a9ae339 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -18703,10 +18703,18 @@ static void print_verification_stats(struct bpf_verifier_env *env)
>>  	if (env->log.level & BPF_LOG_STATS) {
>>  		verbose(env, "verification time %lld usec\n",
>>  			div_u64(env->verification_time, 1000));
>> -		verbose(env, "stack depth %d", env->subprog_info[0].stack_depth);
>> -		for (i = 1; i < subprog_cnt; i++)
>> -			verbose(env, "+%d", env->subprog_info[i].stack_depth);
>> -		verbose(env, " max %d\n", env->max_stack_depth);
>> +		verbose(env, "stack depth");
>> +		for (i = 0; i < subprog_cnt; i++) {
>> +			const char *name = env->subprog_info[i].name;
>> +
>> +			if (name && name[0])
>> +				verbose(env, " %s=%d", name,
>> +					env->subprog_info[i].stack_depth);
>> +			else
>> +				verbose(env, " subprog[%d]=%d", i,
>> +					env->subprog_info[i].stack_depth);
>
> That would be not very convenient to parse by veristat,
> but I'd print every subprogram on it's own line.
>

I can do that, but let's agree on the format before I get to it.
How should it look like? Prefix with stack depth every line, and print stack
depth max ... as the very first line? Like so:

stack depth max N
stack depth subprog 0 <name> N
...
stack depth subprog N <name> N

?

>> +		}
>> +		verbose(env, " max=%d\n", env->max_stack_depth);
>
> I think check_max_stack_depth_subprog() needs a similar update,
> at the moment it reports a not very helpful error like this:
>
>   combined stack size of 34 calls is 528. Too large
>
> Printing the offending spine would be helpful.
>

I'll do that in verifier errors patch set. This is just for the verification
stats, that are outside its scope.

> ...