Re: [PATCH 7/9] bpf: Compile the BTF id whitelist data in vmlinux

Jiri Olsa
2020-06-02T08:16:53Z

On Mon, Jun 01, 2020 at 12:06:34PM -0700, Andrii Nakryiko wrote:
> On Sun, May 31, 2020 at 8:10 AM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Fri, May 29, 2020 at 01:48:58PM -0700, Andrii Nakryiko wrote:
> > > On Thu, May 28, 2020 at 10:24 AM Jiri Olsa <jolsa@redhat.com> wrote:
> > > >
> > > > On Thu, May 14, 2020 at 03:46:26PM -0700, Andrii Nakryiko wrote:
> > > >
> > > > SNIP
> > > >
> > > > > > I was thinking of putting the names in __init section and generate the BTF
> > > > > > ids on kernel start, but the build time generation seemed more convenient..
> > > > > > let's see the linking times with 'real size' whitelist and we can reconsider
> > > > > >
> > > > >
> > > > > Being able to record such places where to put BTF ID in code would be
> > > > > really nice, as Alexei mentioned. There are many potential use cases
> > > > > where it would be good to have BTF IDs just put into arbitrary
> > > > > variables/arrays. This would trigger compilation error, if someone
> > > > > screws up the name, or function is renamed, or if function can be
> > > > > compiled out under some configuration. E.g., assuming some reasonable
> > > > > implementation of the macro
> > > >
> > > > hi,
> > > > I'm struggling with this part.. to get some reasonable reference
> > > > to function/name into 32 bits? any idea? ;-)
> > > >
> > >
> > > Well, you don't have to store actual pointer, right? E.g, emitting
> > > something like this in assembly:
> > >
> > > .global __BTF_ID___some_function
> > > .type __BTF_ID___some_function, @object
> > > .size __BTF_ID___some_function, 4
> > > __BTF_ID___some_function:
> > > .zero 4
> > >
> > > Would reserve 4 bytes and emit __BTF_ID___some_function symbol. If we
> > > can then post-process vmlinux image and for all symbols starting with
> > > __BTF_ID___ find some_function BTF type id and put it into those 4
> > > bytes, that should work, no?
> > >
> > > Maybe generalize it to __BTF_ID__{func,struct,typedef}__some_function,
> > > whatever, not sure. Just an idea.
> >
> > nice, so something like below?
> >
> > it'd be in .S file, or perhaps in inline asm, assuming I'll be
> > able to pass macro arguments to asm("")
>
> I'd do inline asm, there are no arguments you need to pass into
> asm("") itself, everything can be done through macro string
> interpolation, I think. Having everything in .c file would be way more
> convenient and obvious.

wil will do it in inline asm

thanks,
jirka