Re: duplicate BTF_IDs leading to symbol redefinition errors?

Masahiro Yamada
2023-09-14T08:31:29Z

On Thu, Sep 14, 2023 at 5:17 PM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Fri, Sep 08, 2023 at 10:15:35PM +0200, Jiri Olsa wrote:
> > On Fri, Sep 08, 2023 at 10:14:56AM -0700, Nick Desaulniers wrote:
> > > Thanks for the patch!
> > >
> > > + Marcus
> > >
> > > Marcus can you please test the below patch and provide your tested-by
> > > and reported-by tags?
> > >
> > > On Fri, Sep 8, 2023 at 4:47 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> > > >
> > > > On Thu, Sep 07, 2023 at 10:33:00PM +0200, Jiri Olsa wrote:
> > > > > On Thu, Sep 07, 2023 at 12:01:18PM -0700, Nick Desaulniers wrote:
> > > > > > So we've got a curious report recently:
> > > > > > https://github.com/ClangBuiltLinux/linux/issues/1913
> > > > > >
> > > > > > ld.lld: error: ld-temp.o <inline asm>:14577:1: symbol
> > > > > > '__BTF_ID__struct__cgroup__624' is already defined
> > > > > > __BTF_ID__struct__cgroup__624:
> > > > > > ^
> > > > > >
> > > > > > It's been hard to pin down a SHA and .config to reproduce this, but
> > > > > > looking at the definition of BTF_ID's usage of __ID's usage of
> > > > > > __COUNTER__, and the two statements:
> > > > > >
> > > > > > kernel/bpf/helpers.c:2460:BTF_ID(struct, cgroup)
> > > > > > kernel/bpf/verifier.c:5075:BTF_ID(struct, cgroup)
> > > > > >
> > > > > > Is it possible that __COUNTER__ could evaluate to the same value
> > > > > > across 2 different translation units, leading to a name collision like
> > > > > > the above?
> > > > >
> > > > > hum, that probably the case, I see same counter values at different
> > > > > __BTF_ID_ symbols:
> > > > >
> > > > > ffffffff833fe540 r __BTF_ID__struct__bpf_bloom_filter__380
> > > > > ffffffff833fe548 r __BTF_ID__struct__bpf_queue_stack__380
> > > > > ffffffff833fe578 r __BTF_ID__struct__cgroup__380
> > > > >
> > > > > perhaps we were just lucky not to hit that :-\
> > > > >
> > > > > >
> > > > > > looking at another usage of BTF_ID other than struct
> > > > > > cgroup;kernel/bpf/helpers.c:2461:BTF_ID(func, bpf_cgroup_release)
> > > > > > is only defined in one translation unit
> > > > > >
> > > > > > Should one of those two `BTF_ID(struct, cgroup)` be removed? Is there
> > > > > > some other way we can avoid these collisions in the future?
> > > > >
> > > > > need to find some way to make the symbol unique, will check
> > > >
> > > > the change below uses object's path as the __BTF_ID_.. symbol suffix to make
> > > > it unique
> > > >
> > > > I'm still looking, but can't think of a better way so far, perhaps somebody
> > > > will have better idea
> > >
> > > Another good approach; I had simply added __LINE__ into the paste.
> > > https://github.com/ClangBuiltLinux/linux/issues/1913#issuecomment-1710794319
> > > Which just makes the probability of this occurring again smaller, but
> > > still non-zero.
> >
> > yes, there's still possibility of the match
> >
> > >
> > > + Masahiro for thoughts on the invocation of echo and base32. Looks
> > > like base32 is part of coreutils. Kind of strange that coreutils isn't
> > > listed in Documentation/process/changes.rst. Would adding the usage
> > > of base32 add a new dependency on coreutils?
> > >
> > > >
> > > > jirka
> > > >
> > > >
> > > > ---
> > > > diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> > > > index a3462a9b8e18..564953f9cbc7 100644
> > > > --- a/include/linux/btf_ids.h
> > > > +++ b/include/linux/btf_ids.h
> > > > @@ -49,7 +49,7 @@ word \
> > > > ____BTF_ID(symbol, word)
> > > >
> > > > #define __ID(prefix) \
> > > > - __PASTE(prefix, __COUNTER__)
> > > > + __PASTE(__PASTE(prefix, __COUNTER__), BTF_ID_BASE)
> > >
> > > Do we still need __COUNTER__ if we're now using BTF_ID_BASE?
> >
> > yes we still need that because we could have same __BTF_ID__...
> > symbol used multiple times within same object, and that's where
> > __COUNTER__ makes the difference
> >
> > >
> > > >
> > > > /*
> > > > * The BTF_ID defines unique symbol for each ID pointing
> > > > diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> > > > index 68d0134bdbf9..2ef8b2798be0 100644
> > > > --- a/scripts/Makefile.lib
> > > > +++ b/scripts/Makefile.lib
> > > > @@ -200,6 +200,10 @@ _c_flags += $(if $(patsubst n%,, \
> > > > -D__KCSAN_INSTRUMENT_BARRIERS__)
> > > > endif
> > > >
> > > > +ifeq ($(CONFIG_DEBUG_INFO_BTF),y)
> > > > +_c_flags += -DBTF_ID_BASE=$(subst =,,$(shell echo -n $(modfile) | base32 -w0))
> > >
> > > `man 1 base32` shows it can just read a file. Could the above be:
> > >
> > > _c_flags += -DBTF_ID_BASE=$(subst =,,$(shell base32 -w0 $(modfile)))
> > >
> > > ? (untested)
> > >
> > > Also, the output of
> > >
> > > $ base32 -w0 Documentation/process/changes.rst
> > >
> > > is 24456 characters. This is going to blow up symbol tables. I
> > > suppose ELF probably has some length limit on symbol names, too. I
> > > was nervous about my approaching appending __LINE__.
> > >
> > > Perhaps pipe the output to `head -c <n bytes>`?
> >
> > 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.

Better to consult BTF forks.

--
Best Regards
Masahiro Yamada