Re: duplicate BTF_IDs leading to symbol redefinition errors?

Andrii Nakryiko
2023-09-14T18:14:33Z

On Thu, Sep 14, 2023 at 2:52 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Thu, Sep 14, 2023 at 05:30:51PM +0900, Masahiro Yamada wrote:
>
> SNIP
>
> > > > so the change is about adding unique id that's basically path of
> > > > the object stored in base32 so it could be used as symbol, so we
> > > > don't really need to read the actual file
> > > >
> > > > the problem is when BTF_ID definition like:
> > > >
> > > > BTF_ID(struct, cgroup)
> > > >
> > > > translates in 2 separate objects into same symbol name because of
> > > > the matching __COUNTER__ macro values (like 380 below)
> > > >
> > > > __BTF_ID__struct__cgroup__380
> > > >
> > > > this change just adds unique id of the path name at the end of the
> > > > symbol with:
> > > >
> > > > echo -n 'kernel/bpf/helpers' | base32 -w0 --> NNSXE3TFNQXWE4DGF5UGK3DQMVZHG
> > > >
> > > > so the symbol looks like:
> > > >
> > > > __BTF_ID__struct__cgroup__380NNSXE3TFNQXWE4DGF5UGK3DQMVZHG
> > > >
> > > > and is unique over the sources
> > > >
> > > > but I still hope we could come up with some better solution ;-)
> > >
> > > so far the only better solution I could come up with is to use
> > > cksum (also from coreutils) instead of base32, which makes the
> > > BTF_ID_BASE value compact
> > >
> > > I'll run test to find out how much it hurts the build time
> > >
> > > jirka
> >
> >
> >
> > Seems a bad idea to me.
> >
> > It would fork a new shell and chsum for all files,
> > while only a few of them need it.
>
> right, I have a change to limit this on kernel and net directories,
> but it's still bad
>
> >
> > Better to consult BTF forks.
>
> perhaps there's better way within kbuild to get unique id/value
> for each object file?

let's just use __LINE__ + __COUNTER__ for now and teach resolve_btfids
to fail and complain loudly about duplicate symbols?

This will give us time and opportunity to implement a better approach
to .BTF_ids overall. Encoding the desired type name in the symbol name
always felt off. Maybe it's better to encode type + name as data,
which is discarded at the latest stage during vmlinux linking? Either
way, this baseid hack seems worse and unnecessary.

>
> thanks,
> jirka
>