linux-kernel - Re: [PATCH bpf] bpf/btf: Accept function names that contain dots

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Date:   Tue, 20 Jun 2023 10:38:38 -0400
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Yonghong Song <yhs@...a.com>
Cc:     Florent Revest <revest@...omium.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, martin.lau@...ux.dev, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        nathan@...nel.org, trix@...hat.com, stable@...r.kernel.org
Subject: Re: [PATCH bpf] bpf/btf: Accept function names that contain dots

On Mon, Jun 19, 2023 at 2:17 PM Yonghong Song <yhs@...a.com> wrote:
>
> How many people really build the kernel with
>     LLVM=1 LLVM_IAS=0
> which uses clang compiler ans gcc 'as'.
> I think distro most likely won't do this if they intend to
> build the kernel with clang.
>
> Note that
>     LLVM=1
> implies to use both clang compiler and clang assembler.

Yes, we prefer folks to build with LLVM=1.  The problem exists for
users of stable kernels that predate LLVM_IAS=1 support working well
(4.19 is when we had most of the assembler related issues sorted out,
actually later but we backported most fixes to 4.19).

>
> Using clang17 and 'LLVM=1 LLVM_IAS=0', with latest bpf-next,
> I actually hit some build errors like:
>
> /tmp/video-bios-59fa52.s: Assembler messages:
> /tmp/video-bios-59fa52.s:4: Error: junk at end of line, first
> unrecognized character is `"'

Probably because:
1. CONFIG_DEBUG_INFO_DWARF5=y was set or
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y and you're using a version
of clang which implicitly defaults to DWARFv5.
2. you're using a version of GAS that does not understand DWARFv5.
3. you did not run defconfig/menuconfig to have kconfig check for
DWARFv5 support.

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 unrecognized
> 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 unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:6: Error: junk at end of line, first unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:7: Error: junk at end of line, first unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:8: Error: junk at end of line, first unrecognized
> 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 arbitrary
> 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.

> 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=0 or
> hack the kernel source itself.



-- 
Thanks,
~Nick Desaulniers