Re: [PATCH bpf-next v3 3/4] libbpf: Request verifier warnings for object loads

Alexei Starovoitov
2026-04-20T15:49:56Z

On Mon Apr 20, 2026 at 8:23 AM PDT, Kumar Kartikeya Dwivedi wrote:
> On Mon, 20 Apr 2026 at 15:57, Mykyta Yatsenko
> <mykyta.yatsenko5@gmail.com> wrote:
>>
>> On 4/18/26 6:16 PM, Kumar Kartikeya Dwivedi wrote:
>> > On kernels supporting BPF_LOG_LEVEL_WARN, have libbpf request verifier
>> > warnings for BPF object program loads by ORing in the warning bit. This
>> > keeps the existing log_level=0 retry semantics for verbose logs, while
>> > still surfacing verifier warnings for successful loads.
>> >
>> > When libbpf owns the log buffer, use a small buffer for warning-only
>> > loads and flush captured warnings through the print callback. Detect
>> > support by probing for BPF_FEAT_VERIFIER_WARNINGS in enum bpf_features,
>> > and document the updated kernel_log_level behavior.
>> >
>> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>> > ---
>> [...]
>> > @@ -7954,7 +7960,10 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
>> >
>> > ret = bpf_prog_load(prog->type, prog_name, license, insns, insns_cnt, &load_attr);
>> > if (ret >= 0) {
>> > - if (log_level && own_log_buf) {
>> > + if (log_level == 16 && load_attr.log_true_size && own_log_buf) {
>> > + pr_warn("prog '%s': -- BEGIN PROG LOAD WARNINGS --\n%s-- END PROG LOAD WARNINGS --\n",
>> > + prog->name, log_buf);
>>
>> I'm not sure if this block makes sense to me: we only print warnings
>> banner is warnings is the only requested log type, but we print no
>> banner if warnings are requested with other log level, also different
>> log_levels: pr_warn() vs pr_debug().
>>
>
> With other log levels, it becomes part of regular verifier output, if
> requested. By default, we want to print them on success at a higher
> log level upon success so they appear, even if the user didn't request
> verbose logs in the log buffer.
> Since the default case only generates warnings, we change the banner,
> but keep it as-is for other scenarios. Does that make sense?

I'm not a fan of banner either.
Screaming BEGIN and END is imo too verbose. I would skip it altogether.
And probably would have removed it from existing 2 places in libbpf.