Re: [PATCH bpf-next v1 3/3] libbpf: Wire up verifier warning display logic

Andrii Nakryiko
2026-03-30T23:46:43Z

On Sun, Mar 29, 2026 at 2:25 PM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> Wire up the flushing of all accumulated messages in a program's stderr
> stream after verification is complete. An example is shown below of a
> warning printed now about usage of deprecated kfuncs.
>
> $ ./test_progs -t kfunc_implicit_args/test_kfunc_implicit_arg_legacy_impl -v
> ...
> libbpf: prog 'test_kfunc_implicit_arg_legacy_impl': VERIFIER WARNINGS:
> WARNING: kfunc_implicit_args.c:40 calls deprecated kfunc bpf_kfunc_implicit_arg_legacy_impl(), which will be removed.
> WARNING: Switch to kfunc bpf_kfunc_implicit_arg_legacy() instead.
> WARNING: For older kernels, choose the kfunc using bpf_ksym_exists(bpf_kfunc_implicit_arg_legacy).
> ...
>
> Note that test_progs overrides the default logging function, hence -v is
> necessary. By default these messages would be printed for the user.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
> tools/lib/bpf/libbpf.c | 23 +++++++++++++++++++++++
> 1 file changed, 23 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 9ea41f40dc82..f308bacd083f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -7826,6 +7826,27 @@ static int libbpf_prepare_prog_load(struct bpf_program *prog,
>
> static void fixup_verifier_log(struct bpf_program *prog, char *buf, size_t buf_sz);
>
> +static void bpf_object_load_prog_emit_stderr(struct bpf_program *prog, int prog_fd)
> +{
> + char chunk[256];
> + bool emitted = false;
> + int ret;
> +
> + while ((ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDERR, chunk,
> + sizeof(chunk), NULL)) > 0) {
> + if (!emitted) {
> + pr_warn("prog '%s': VERIFIER WARNINGS:\n", prog->name);
> + emitted = true;
> + }
> + libbpf_print(LIBBPF_WARN, "%.*s", ret, chunk);
> + }
> +
> + if (ret < 0) {
> + pr_debug("prog '%s': failed to read BPF stderr stream: %s\n",
> + prog->name, errstr(ret));

instead of potentially spamming debug logs with this, let's detect if
streams are supported and not even try to read warnings?

Also, why 256 byte buffer? 1-4KB isn't a big deal, it's user space

> + }
> +}
> +
> static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog,
> struct bpf_insn *insns, int insns_cnt,
> const char *license, __u32 kern_version, int *prog_fd)
> @@ -7950,6 +7971,8 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
> prog->name, log_buf);
> }
>
> + bpf_object_load_prog_emit_stderr(prog, ret);

just inline this, we inline bind map logic, so why not this one?

> +
> if (obj->has_rodata && kernel_supports(obj, FEAT_PROG_BIND_MAP)) {
> struct bpf_map *map;
> int i;
> --
> 2.52.0
>