linux-kernel - Re: [PATCH bpf-next v8 3/5] libbpf: Add libbpf_probe_bpf

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Message-ID: <CAEf4BzYr9WzYbmyq8=nVETDqYvmYmObhD6x+_TQYpSUWxxGLLg@mail.gmail.com>
Date: Mon, 24 Feb 2025 17:15:49 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, eddyz87@...il.com, 
	haoluo@...gle.com, jolsa@...nel.org, qmo@...nel.org, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, chen.dylane@...il.com, 
	Tao Chen <dylane.chen@...iglobal.com>
Subject: Re: [PATCH bpf-next v8 3/5] libbpf: Add libbpf_probe_bpf_kfunc API

On Mon, Feb 24, 2025 at 9:02 AM Tao Chen <chen.dylane@...ux.dev> wrote:
>
> Similarly to libbpf_probe_bpf_helper, the libbpf_probe_bpf_kfunc
> used to test the availability of the different eBPF kfuncs on the
> current system.
>
> Cc: Tao Chen <dylane.chen@...iglobal.com>
> Reviewed-by: Jiri Olsa <jolsa@...nel.org>
> Reviewed-by: Eduard Zingerman <eddyz87@...il.com>
> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
> ---
>  tools/lib/bpf/libbpf.h        | 19 ++++++++++++-
>  tools/lib/bpf/libbpf.map      |  1 +
>  tools/lib/bpf/libbpf_probes.c | 51 +++++++++++++++++++++++++++++++++++
>  3 files changed, 70 insertions(+), 1 deletion(-)
>

[...]

> +       buf[0] = '\0';
> +       ret = probe_prog_load(prog_type, insns, insn_cnt, btf_fd >= 0 ? fd_array : NULL,
> +                             buf, sizeof(buf));
> +       if (ret < 0)
> +               return libbpf_err(ret);
> +
> +       if (ret > 0)
> +               return 1; /* assume supported */
> +
> +       /* If BPF verifier recognizes BPF kfunc but it's not supported for
> +        * given BPF program type, it will emit "calling kernel function
> +        * <name> is not allowed". If the kfunc id is invalid,
> +        * it will emit "kernel btf_id <id> is not a function". If BTF fd
> +        * invalid in module BTF, it will emit "invalid module BTF fd specified" or
> +        * "negative offset disallowed for kernel module function call". If
> +        * kfunc prog not dev buound, it will emit "metadata kfuncs require
> +        * device-bound program".
> +        */
> +       if (strstr(buf, "not allowed") || strstr(buf, "not a function") ||
> +          strstr(buf, "invalid module BTF fd") ||

why is invalid module BTF FD not an error (negative return)?

> +          strstr(buf, "negative offset disallowed") ||
> +          strstr(buf, "device-bound program"))
> +               return 0;
> +
> +       return 1;
> +}
> +
>  int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helper_id,
>                             const void *opts)
>  {
> --
> 2.43.0
>