Re: duplicate BTF_IDs leading to symbol redefinition errors?

[Jiri Olsa <olsajiri@gmail.com>]

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..

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

> way, this baseid hack seems worse and unnecessary.

yes, it's bad

jirka