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

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Date:   Tue, 20 Jun 2023 07:53:28 -0700
From:   Yonghong Song <yhs@...a.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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 6/20/23 7:38 AM, Nick Desaulniers wrote:
> 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.

Yes, I am using llvm17 which I think the default dwarf is v5, and
indeed I have
   CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y

My gas is 2.30, probably too old to support dwarf 5.

Yes, I didn't run defconfig/menuconfig.

> 
> 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.

Thanks Florent pointing out 5.10 stable kernels which have this issue.
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=0 or
>> hack the kernel source itself.
> 
> 
>