Re: [PATCH bpf-next v5 1/2] bpf: Only enforce 8 frame call stack limit for all-static stacks

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Mar 11, 2026 at 3:16 PM EDT, bot+bpf-ci wrote:
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 4fbacd2149cd..0304e4329490 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -6733,9 +6733,11 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
>>  	struct bpf_insn *insn = env->prog->insnsi;
>>  	int depth = 0, frame = 0, i, subprog_end, subprog_depth;
>>  	bool tail_call_reachable = false;
>> -	int ret_insn[MAX_CALL_FRAMES];
>> -	int ret_prog[MAX_CALL_FRAMES];
>> -	int j;
>> +	int total;
>> +	int tmp;
>> +
>> +	/* no caller idx */
>> +	env->dinfo[idx].caller = -1;
>
> The function header comment above this hunk still reads:
>
>     Since recursion is prevented by check_cfg() this algorithm
>     only needs a local stack of MAX_CALL_FRAMES to remember callsites
>
> The local stack arrays (ret_insn[] and ret_prog[]) have been removed
> in favor of per-subprog dinfo fields. Should this comment be updated
> to reflect the new approach?
>
> This was also noted during the v3 and v4 reviews:
> https://lore.kernel.org/bpf/ad397307a9cd76c9739ee05f4642c7b6a881b7dfb8a2facefa6b2062ffe328ea@mail.kernel.org/
>
> [ ... ]
>
>> diff --git a/tools/testing/selftests/bpf/progs/test_global_func3.c b/tools/testing/selftests/bpf/progs/test_global_func3.c
>> index 142b682d3c2f..974fd8c19561 100644
>> --- a/tools/testing/selftests/bpf/progs/test_global_func3.c
>> +++ b/tools/testing/selftests/bpf/progs/test_global_func3.c
>
> [ ... ]
>
>>  SEC("tc")
>> -__failure __msg("the call stack of 8 frames")
>> +__failure __msg("the call stack of 9 frames")
>>  int global_func3(struct __sk_buff *skb)
>>  {
>>  	return f8(skb);
>
> Should this be "the call stack of 8 frames" rather than 9?
>
> The test has 9 subprogs total (global_func3 plus f1 through f8), but
> only 8 call levels. The entry function global_func3 starts at frame 0
> and is not itself counted by the frame increment. Each static call
> increments frame by one:
>
>     check_max_stack_depth_subprog() {
>         ...
>         frame = subprog_is_global(env, idx) ? 0 : frame + 1;
>         if (frame >= MAX_CALL_FRAMES) {
>             verbose(env, "the call stack of %d frames is too deep !\n",
>                 frame);
>
> With 8 static calls (global_func3 -> f8 -> f7 -> ... -> f1), frame
> reaches 8, which matches MAX_CALL_FRAMES. The verifier would print
> "the call stack of 8 frames is too deep" and the __msg substring
> check for "9 frames" would not match, causing the test to fail.
>
> The old test with global functions also had 8 call levels and
> expected "8 frames", since frame++ produced the same count of 8.
>

I'd say no to either suggestion. The updated expected failure message is
correct, and the old comment still applies for consecutive static stack
frames even if we don't refer to a single stack-allocated call stack anymore.

>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/22968848938