linux-kernel - Re: [PATCH bpf-next v3 2/2] bpf: Refactor bpf_tracing_func_proto() and remove bpf_get_probe_write

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Message-ID: <CAADnVQL6yyRRUc1Xee4HOQ0QXEiqQ7M-xJ109w9aztYH4ZWHmA@mail.gmail.com>
Date: Thu, 28 Nov 2024 10:22:42 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Marco Elver <elver@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Nikola Grcevski <nikola.grcevski@...fana.com>, 
	bpf <bpf@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 2/2] bpf: Refactor bpf_tracing_func_proto()
 and remove bpf_get_probe_write_proto()

On Wed, Nov 27, 2024 at 6:10 AM Marco Elver <elver@...gle.com> wrote:
>
> With bpf_get_probe_write_proto() no longer printing a message, we can
> avoid it being a special case with its own permission check.
>
> Refactor bpf_tracing_func_proto() similar to bpf_base_func_proto() to
> have a section conditional on bpf_token_capable(CAP_SYS_ADMIN), where
> the proto for bpf_probe_write_user() is returned. Finally, remove the
> unnecessary bpf_get_probe_write_proto().
>
> This simplifies the code, and adding additional CAP_SYS_ADMIN-only
> helpers in future avoids duplicating the same CAP_SYS_ADMIN check.
>
> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Marco Elver <elver@...gle.com>
> Acked-by: Jiri Olsa <jolsa@...nel.org>
> ---
> v3:
> * Fix where bpf_base_func_proto() is called - it needs to be last,
>   because we may override protos (as is e.g. done for
>   BPF_FUNC_get_smp_processor_id).
>
> v2:
> * New patch.
> ---
>  kernel/trace/bpf_trace.c | 25 +++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 0ab56af2e298..9b1d1fa4c06c 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -357,14 +357,6 @@ static const struct bpf_func_proto bpf_probe_write_user_proto = {
>         .arg3_type      = ARG_CONST_SIZE,
>  };
>
> -static const struct bpf_func_proto *bpf_get_probe_write_proto(void)
> -{
> -       if (!capable(CAP_SYS_ADMIN))
> -               return NULL;
> -
> -       return &bpf_probe_write_user_proto;
> -}
> -
>  #define MAX_TRACE_PRINTK_VARARGS       3
>  #define BPF_TRACE_PRINTK_SIZE          1024
>
> @@ -1458,9 +1450,6 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>                 return &bpf_perf_event_read_proto;
>         case BPF_FUNC_get_prandom_u32:
>                 return &bpf_get_prandom_u32_proto;
> -       case BPF_FUNC_probe_write_user:
> -               return security_locked_down(LOCKDOWN_BPF_WRITE_USER) < 0 ?
> -                      NULL : bpf_get_probe_write_proto();
>         case BPF_FUNC_probe_read_user:
>                 return &bpf_probe_read_user_proto;
>         case BPF_FUNC_probe_read_kernel:
> @@ -1539,8 +1528,20 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>         case BPF_FUNC_trace_vprintk:
>                 return bpf_get_trace_vprintk_proto();
>         default:
> -               return bpf_base_func_proto(func_id, prog);
> +               break;
>         }
> +
> +       if (bpf_token_capable(prog->aux->token, CAP_SYS_ADMIN)) {
> +               switch (func_id) {
> +               case BPF_FUNC_probe_write_user:
> +                       return security_locked_down(LOCKDOWN_BPF_WRITE_USER) < 0 ?
> +                              NULL : &bpf_probe_write_user_proto;
> +               default:
> +                       break;
> +               }
> +       }
> +
> +       return bpf_base_func_proto(func_id, prog);

Moving bpf_base_func_proto() all the way to the top was incorrect,
but here we can move it just above this bpf_token_capable() check
and remove extra indent like:

func_proto = bpf_base_func_proto();
if (func_proto)
   return func_proto;
if (!bpf_token_capable(prog->aux->token, CAP_SYS_ADMIN))
   return NULL;
switch (func_id) {
case BPF_FUNC_probe_write_user:

that will align it with the style of bpf_base_func_proto().

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