Re: [PATCH bpf-next 1/3] bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: David Vernet <void@manifault.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>, Yonghong Song <yhs@meta.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Kernel Team <kernel-team@meta.com>
Subject: Re: [PATCH bpf-next 1/3] bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs
Date: Fri, 6 Jan 2023 17:04:02 -0800	[thread overview]
Message-ID: <CAADnVQLpK7WXTjF6GS1hcfPXf=8iERJmEeVFfvmG75mJj0DdaA@mail.gmail.com> (raw)
In-Reply-To: <20230106195130.1216841-2-void@manifault.com>

On Fri, Jan 6, 2023 at 11:51 AM David Vernet <void@manifault.com> wrote:
>
> kfuncs are functions defined in the kernel, which may be invoked by BPF
> programs. They may or may not also be used as regular kernel functions,
> implying that they may be static (in which case the compiler could e.g.
> inline it away), or it could have external linkage, but potentially be
> elided in an LTO build if a function is observed to never be used, and
> is stripped from the final kernel binary.
>
> We therefore require some convenience macro that kfunc developers can
> use just add to their kfuncs, and which will prevent all of the above
> issues from happening. This is in contrast with what we have today,
> where some kfunc definitions have "noinline", some have "__used", and
> others are static and have neither.
>
> In addition to providing the obvious correctness benefits, having such a
> macro / tag also provides the following advantages:
>
> - Giving an easy and intuitive thing to query for if people are looking
>   for kfuncs, as Christoph suggested at the kernel maintainers summit
>   (https://lwn.net/Articles/908464/). This is currently possible by
>   grepping for BTF_ID_FLAGS(func, but having something more self
>   describing would be useful as well.
>
> - In the future, the tag can be expanded with other useful things such
>   as the ability to suppress -Wmissing-prototype for the kfuncs rather
>   than requiring developers to surround the kfunc with __diags to
>   suppress the warning (this requires compiler support that as far as I
>   know currently does not exist).

Have you considered doing bpf_kfunc_start/bpf_kfunc_end ?
The former would include:
__diag_push(); __diag_ignore_all(); __used noinline

Also how about using bpf_kfunc on the same line ?
Then 'git grep' will be easier.