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

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Mon, Jan 09, 2023 at 04:47:54AM IST, Alexei Starovoitov wrote:
> On Fri, Jan 6, 2023 at 6:09 PM David Vernet <void@manifault.com> wrote:
> >
> > On Fri, Jan 06, 2023 at 05:04:02PM -0800, Alexei Starovoitov wrote:
> > > 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
> >
> > Yeah that's certainly an option. The downside is that all functions
> > within scope of the __diag_push() will be affected, and sometimes we mix
> > kfuncs with non-kfuncs (including e.g. static helper functions that are
> > used by the kfuncs themselves). -Wmissing-prototypes isn't a big deal,
> > but __used and noinline are kind of unfortunate. Not a big deal though,
> > it'll just result in a few extra __bpf_kfuncs_start() and
> > __bpf_kfuncs_end() sprinkled throughout to avoid them being included.
> > The upside is of course that we can get rid of the __diag_push()'es we
> > currently have to prevent -Wmissing-prototypes.
>
> I meant to use bpf_kfunc_start/bpf_kfunc_end around every kfunc.
> Ideally bpf_kfunc_start would be on the same line as func proto
> for nice grepping.
> Maybe it's an overkill.
> Maybe 3 macroses then?
> bpf_kfunc_start to hide __diag
> bpf_kfunc on the proto line
> bpf_kfunc_end to finish __diag_pop
>

There's also the option of doing this:

#define BPF_KFUNC(proto) proto; __used noinline proto

BPF_KFUNC(void kfunc(arg1, arg2)) {
	...
}

No need to disable the warning with diag push/pop, just put a declaration before
the definition to silence the compiler. The only awkward part is entire function
prototype becoming a macro argument (unlike the common case void MACRO(...)) but
it becomes less noisy and easy to grep as well.