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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 20 Jun 2023 11:07:10 -0400
Message-ID: <CAKwvOdna=1Sg4Aab=BE6F86H9ZE7kPRM=VTkqQuGiF-Jdze-cA@mail.gmail.com>
Subject: Re: [PATCH bpf] bpf/btf: Accept function names that contain dots
To:     Yonghong Song <yhs@meta.com>
Cc:     Florent Revest <revest@chromium.org>,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev, martin.lau@linux.dev, ast@kernel.org,
        daniel@iogearbox.net, andrii@kernel.org, song@kernel.org,
        yhs@fb.com, john.fastabend@gmail.com, kpsingh@kernel.org,
        sdf@google.com, haoluo@google.com, jolsa@kernel.org,
        nathan@kernel.org, trix@redhat.com, stable@vger.kernel.org
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On Tue, Jun 20, 2023 at 10:53=E2=80=AFAM Yonghong Song <yhs@meta.com> wrote=
:
>
>
>
> On 6/20/23 7:38 AM, Nick Desaulniers wrote:
> > 3. you did not run defconfig/menuconfig to have kconfig check for
> > DWARFv5 support.
>
> Yes, I didn't run defconfig/menuconfig.

That doesn't mean the odd combo of clang+gas doesn't work.

Just like how using scripts/config is a hazard since it also doesn't
run kconfig and allows you to set incompatible configurations. Garbage
in; garbage out.

>
> >
> > The kconfigs should prevent you from selecting DWARFv5 if your
> > toolchain combination doesn't support it; if you run kconfig.
> >
> >> /tmp/video-bios-59fa52.s:4: Error: file number less than one
> >> /tmp/video-bios-59fa52.s:5: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> /tmp/video-bios-59fa52.s:6: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> /tmp/video-bios-59fa52.s:7: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> /tmp/video-bios-59fa52.s:8: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> /tmp/video-bios-59fa52.s:9: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> /tmp/video-bios-59fa52.s:10: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> /tmp/video-bios-59fa52.s:68: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> clang: error: assembler command failed with exit code 1 (use -v to see
> >> invocation)
> >> make[4]: *** [/home/yhs/work/bpf-next/scripts/Makefile.build:252:
> >> arch/x86/realmode/rm/video-bios.o] Error 1
> >> make[4]: *** Waiting for unfinished jobs....
> >> /tmp/wakemain-88777c.s: Assembler messages:
> >> /tmp/wakemain-88777c.s:4: Error: junk at end of line, first unrecogniz=
ed
> >> character is `"'
> >> /tmp/wakemain-88777c.s:4: Error: file number less than one
> >> /tmp/wakemain-88777c.s:5: Error: junk at end of line, first unrecogniz=
ed
> >> character is `"'
> >> /tmp/wakemain-88777c.s:6: Error: junk at end of line, first unrecogniz=
ed
> >> character is `"'
> >> /tmp/wakemain-88777c.s:7: Error: junk at end of line, first unrecogniz=
ed
> >> character is `"'
> >> /tmp/wakemain-88777c.s:8: Error: junk at end of line, first unrecogniz=
ed
> >> character is `"'
> >> /tmp/wakemain-88777c.s:81: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> /tmp/wakemain-88777c.s:312: Error: junk at end of line, first
> >> unrecognized character is `"'
> >> clang: error: assembler command failed with exit code 1 (use -v to see
> >> invocation)
> >>
> >> Potentially because of my local gnu assembler 2.30-120.el8 won't work
> >
> > It's recorded in lib/Kconfig.debug that 2.35.2 is required for DWARFv5
> > support if you're using GAS.  My machine has 2.40.
> >
> >> with some syntax generated by clang. Mixing clang compiler and arbitra=
ry
> >> gnu assembler are not a good idea (see the above example). It might
> >
> > I agree, but for older branches of stable which are still supported,
> > we didn't quite have clang assembler support usable.  We still need to
> > support those branches of stable.
>
> Thanks Florent pointing out 5.10 stable kernels which have this issue.

No, all kernels have this issue, when using `LLVM=3D1 LLVM_IAS=3D0`.  It's
more likely that someone is using that combination for branches of
stable that predate 4.19 (such as 4.14) but we do still try to support
that combination somewhat, even if we recommend just using `LLVM=3D1`.
Interop between toolchains is still important, even if "why would you
do that?"

> I am okay with backporting to old stable kernels if that is needed.
> But the patch going to bpf-next should not have a bug-fix tag and
> the patch commit message can be tweaked for backport to 5.10 though.
>
> >
> >> work with close-to-latest gnu assembler.
> >>
> >> To support function name like '<fname>.isra', some llvm work will be
> >> needed, and it may take some time.
> >>
> >> So in my opinion, this patch is NOT a bug fix. It won't affect distro.
> >> Whether we should backport to the old kernel, I am not sure whether it
> >> is absolutely necessary as casual build can always remove LLVM_IAS=3D0=
 or
> >> hack the kernel source itself.
> >
> >
> >


--=20
Thanks,
~Nick Desaulniers