Re: duplicate BTF_IDs leading to symbol redefinition errors?

Jiri Olsa
2023-09-07T20:33:05Z

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

>
> Was this a previously observed/fixed issue?

first time I see that

thanks,
jirka