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

Alexei Starovoitov
2023-01-07T01:04:22Z

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.