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

[Jiri Olsa <jolsa@redhat.com>]

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? ;-)

jirka

> 
> static const u32 d_path_whitelist[] = {
>     BTF_ID_FUNC(vfs_fallocate),
> #ifdef CONFIG_WHATEVER
>     BTF_ID_FUNC(do_truncate),
> #endif
> };
> 
> Would be nice and very explicit. Given this is not going to be sorted,
> you won't be able to use binary search, but if whitelists are
> generally small, it should be fine as is. If not, hashmap could be
> built in runtime and would be, probably, faster than binary search for
> longer sets of BTF IDs.
> 
> I wonder if we can do some assembly magic with generating extra
> symbols and/or relocations to achieve this? What do you think? Is it
> doable/desirable/better?
> 
> 
> > thanks,
> > jirka
> >
>