Re: duplicate BTF_IDs leading to symbol redefinition errors?

Andrii Nakryiko
2023-09-15T20:41:38Z

On Fri, Sep 15, 2023 at 1:28 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Thu, Sep 14, 2023 at 11:14:19AM -0700, Andrii Nakryiko wrote:
> > 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?
>
> ok, will send that.. but it fails during link before resolve_btfids
> takes place
>
> >
> >
> > 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
>
> hum, so maybe having a special section (.BTF_ids_desc below)
> that would have record for each ID placed in .BTF_ids section:
>
> asm( \
> ".pushsection .BTF_ids,\"a\"; \n" \
> "1: \n" \
> ".zero 4 \n" \
> ".popsection; \n" \
> ".pushsection .BTF_ids_desc,\"a\"; \n" \
> ".long 1b \n" \
>
> and somehow get prefix and name pointers in here:
>
> ".long prefix
> ".long name
>
> ".popsection; \n");
>
> so resolve_btfids would iterate .BTF_ids_desc records and fix
> up .BTF_ids data..
>

Something like that. I don't think it's even a regression in terms of
vmlinux space usage, because right now we spend as much space on
storing symbol names. So just adding string pointers would be already
a win due to repeating "struct", "func", etc strings.

> we might need to do one extra link phase to get rid of the
> .BTF_ids_desc secion

Hopefully we can find a way to avoid this, we already do like 3 link
phases at least (for kallsyms), so doing all that on the first one and
then stripping it out using link script at subsequent one would be
best.

>
> > way, this baseid hack seems worse and unnecessary.
>
> yes, it's bad
>
> jirka