Re: [PATCH bpf-next v3 4/4] selftests/bpf: Test verifier warning logging

Kumar Kartikeya Dwivedi
2026-04-18T20:40:36Z

On Sat, 18 Apr 2026 at 19:45, <sashiko-bot@kernel.org> wrote:
>
> Sashiko AI review found 1 potential issue(s):
> - [Medium] The custom `libbpf_print_cb` unconditionally buffers all log levels into a fixed 4096-byte buffer, which will inevitably overflow during skeleton load and silently drop the expected verifier warnings.
> --
>
> commit 602c54794881684bfce8f29591c7e680ab7d51fe
> Author: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>
> selftests/bpf: Test verifier warning logging
>
> This commit adds test coverage for explicit verifier warning logging and
> libbpf's default warning flush path while keeping the warning bit private
> to the test.
>
> > diff --git a/tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args.c b/tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args.c
> > index 5e4793c9c29a0..f92a27d764557 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args.c
> > @@ -4,7 +4,99 @@
> > #include <test_progs.h>
> > #include "kfunc_implicit_args.skel.h"
> >
> > +#define TEST_BPF_LOG_LEVEL_WARN (1U << 4)
> > +
> > +static size_t libbpf_log_pos;
> > +static char libbpf_log_buf[4096];
> > +static bool libbpf_log_error;
> > +
> > +static int libbpf_print_cb(enum libbpf_print_level level, const char *fmt, va_list args)
> > +{
> > + int emitted_cnt;
> > + size_t left_cnt;
> > +
> > + (void)level;
> > +
> > + left_cnt = sizeof(libbpf_log_buf) - libbpf_log_pos;
> > + emitted_cnt = vsnprintf(libbpf_log_buf + libbpf_log_pos, left_cnt, fmt, args);
> > + if (emitted_cnt < 0 || emitted_cnt + 1 > left_cnt) {
> > + libbpf_log_error = true;
> > + return 0;
> > + }
>
> Since libbpf generates a high volume of LIBBPF_DEBUG and LIBBPF_INFO messages
> during object load, will this unconditionally buffer all log levels and overflow
> the 4096-byte buffer before the load completes?
>
> If the buffer overflows, libbpf_log_error becomes true and subsequent messages
> (including the expected verifier warnings flushed at the end of the load) will
> be discarded.
>
> Should this callback filter out non-warning levels to avoid filling the buffer
> too quickly?

Perhaps, I will check and adjust if necessary.

>
> [ ... ]
>
> > +static void test_default_warning_flush(void)
> > +{
> > + libbpf_print_fn_t old_print_cb = libbpf_set_print(libbpf_print_cb);
> > + struct kfunc_implicit_args *skel = NULL;
> > + int err;
> > +
> > + libbpf_log_pos = 0;
> > + libbpf_log_buf[0] = '\0';
> > + libbpf_log_error = false;
> > +
> > + skel = kfunc_implicit_args__open();
> > + if (!ASSERT_OK_PTR(skel, "skel_open"))
> > + goto cleanup;
> > +
> > + select_legacy_impl_prog(skel);
> > +
> > + err = kfunc_implicit_args__load(skel);
> > + ASSERT_OK(err, "skel_load");
> > + ASSERT_FALSE(libbpf_log_error, "libbpf_log_error");
>
> Will this assertion predictably fail due to the buffer overflow mentioned
> above?

No, it's a non-common case.

>
> > + ASSERT_OK_PTR(strstr(libbpf_log_buf, "-- BEGIN PROG LOAD WARNINGS --"), "warn_banner");
> > + ASSERT_OK_PTR(strstr(libbpf_log_buf,
> > + "uses deprecated kfunc bpf_kfunc_implicit_arg_legacy_impl()"),
> > + "warn_msg");
> > + ASSERT_NULL(strstr(libbpf_log_buf, "-- BEGIN PROG LOAD LOG --"), "no_verbose_log");
> > +
> > +cleanup:
> > + kfunc_implicit_args__destroy(skel);
> > + libbpf_set_print(old_print_cb);
> > +}
>
> --
> Sashiko AI review · https://sashiko.dev/#/patchset/20260418171701.610025-1-memxor@gmail.com?part=4