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

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Message-ID: <2dcc697a-46fe-0933-0508-90ebad9ac8f3@meta.com>
Date:   Tue, 20 Jun 2023 20:28:13 -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 8:07 AM, Nick Desaulniers wrote:
> On Tue, Jun 20, 2023 at 10:53 AM Yonghong Song <yhs@...a.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 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.
> 
> No, all kernels have this issue, when using `LLVM=1 LLVM_IAS=0`.  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=1`.
> Interop between toolchains is still important, even if "why would you
> do that?"

Okay, yes, although 'LLVM=1' is recommended way to compiler clang
based kernel, users can certainly do 'LLVM=1 LLVM_IAS=0' as well
although not recommended. Then it is okay to put a bug fix in
the commit message. Just need to clarify that
   - > 5.10 kernel, LLVM=1 (LLVM_IAS=0 is not the default)
     is recommended but user can still have LLVM=1 LLVM_IAS=0
     to trigger the issue
   - <= 5.10 kernel, LLVM=1 (LLVM_IAS=0 is the default) is
     recommended in which case gnu as will be used.

> 
>> 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.
>>>
>>>
>>>
> 
> 
>