Re: [PATCH dwarves] btf: Generate btf for functions in the .BTF_ids section

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Apr 28, 2021 at 12:45:10PM -0700, Andrii Nakryiko escreveu:
> On Wed, Apr 28, 2021 at 6:10 AM Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com> wrote:
> > Em Tue, Apr 27, 2021 at 01:38:51PM -0700, Andrii Nakryiko escreveu:
> > > On Tue, Apr 27, 2021 at 5:34 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> > > > And tools that expect to trace a function can get that information from
> > > > the BTF info instead of getting some failure when trying to trace those
> > > > functions, right?

> > > I don't think it belongs in BTF, though.

> > My thinking was that since BTF is used when tracing, one would be
> > interested in knowing if some functions can't be used for that.

> > > Plus there are additional limitations enforced by BPF verifier that
> > > will prevent some functions to be attached. So just because the
> > > function is in BTF doesn't mean it's 100% attachable.

> > Well, at least we would avoid some that can't for sure be used for
> > tracing. But, a bit in there is precious, so probably geting a NACK from
> > the kernel should be a good enough capability query. :-)

> > Tools should just silently prune things in wildcards provided by the
> > user that aren't traceable, silently, and provide an error message when
> > the user explicitely asks for tracing a verbotten function.

> Yep, that's what I'm doing in my retsnoop tool (I both filter by
> available kprobes [0], and have extra blacklist [1]). Loading kprobes
> is pretty simple and fast enough, shouldn't be a problem.

>   [0] https://github.com/anakryiko/retsnoop/blob/master/src/mass_attacher.c#L495
>   [1] https://github.com/anakryiko/retsnoop/blob/master/src/mass_attacher.c#L41

Oh my, one more tool to download, build, try, have updated, etc. Will
look at it, thanks for the pointer.

- Arnaldo