linux-kernel - Re: [PATCH bpf-next v3] bpf: Remove redundant checks

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Message-ID: <CAEf4Bzai7NL-3=1SVi4-WWYWEY6Lzrb8GBfKnt6FG8sNm2OMRQ@mail.gmail.com>
Date: Wed, 16 Apr 2025 14:55:43 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Feng Yang <yangfeng59949@....com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, 
	sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, 
	mattbobrowski@...gle.com, rostedt@...dmis.org, mhiramat@...nel.org, 
	mathieu.desnoyers@...icios.com, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v3] bpf: Remove redundant checks

On Thu, Apr 10, 2025 at 12:03 AM Feng Yang <yangfeng59949@....com> wrote:
>
> From: Feng Yang <yangfeng@...inos.cn>
>
> Many conditional checks in switch-case are redundant
> with bpf_base_func_proto and should be removed.
>
> Signed-off-by: Feng Yang <yangfeng@...inos.cn>
> Acked-by: Song Liu <song@...nel.org>
> ---
> Changes in v3:
> - Only modify patch description information.
> - Link to v2: https://lore.kernel.org/all/20250408071151.229329-1-yangfeng59949@163.com/
>
> Changes in v2:
> - Only modify patch description information.
> - Link to v1: https://lore.kernel.org/all/20250320032258.116156-1-yangfeng59949@163.com/
> ---
>  kernel/trace/bpf_trace.c | 72 ----------------------------------------
>  1 file changed, 72 deletions(-)
>

All this looks good, I checked that those functions indeed are allowed
in bpf_base_func_proto. The only (minor) differences are capabilities,
bpf_base_func_proto() correctly guards some of the helpers with
CAP_BPF and/or CAP_PERFMON checks, while bpf_tracing_func_proto()
doesn't seem to bother (which is either a bug or any tracing prog
implies CAP_BPF and CAP_PERFMON, I'm not sure, didn't check).

But I think we can take it further and remove even more stuff from
bpf_tracing_func_proto and/or add more stuff into bpf_base_func_proto
(perhaps as a few patches in a series, so it's easier to review and
validate).

Basically, except for a few custom implementations that depend on
tracing program type (like get_stack and others like that), if
something is OK to call from a tracing program it should be ok to call
from any program type. And as such it can (should?) be added to
bpf_base_func_proto, IMO.

P.S. I'd name the patch/series as "bpf: streamline allowed helpers
between tracing and base sets" or something like that to make the
purpose clearer

[...]

>         case BPF_FUNC_get_current_uid_gid:
>                 return &bpf_get_current_uid_gid_proto;
>         case BPF_FUNC_get_current_comm:
>                 return &bpf_get_current_comm_proto;

I'm surprised these two are not part of bpf_base_func_proto, tbh...
maybe let's move them there while we are cleaning all this up?

pw-bot: cr

> -       case BPF_FUNC_trace_printk:
> -               return bpf_get_trace_printk_proto();
>         case BPF_FUNC_get_smp_processor_id:
>                 return &bpf_get_smp_processor_id_proto;

this one should be cleaned up as well and
bpf_get_smp_processor_id_proto removed. All BPF programs either
disable CPU preemption or CPU migration, so bpf_base_func_proto's
implementation should work just fine (but please do it as a separate
patch)

> -       case BPF_FUNC_get_numa_node_id:
> -               return &bpf_get_numa_node_id_proto;
>         case BPF_FUNC_perf_event_read:
>                 return &bpf_perf_event_read_proto;

[...]