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

[Yonghong Song]

On 6/20/23 7:38 AM, Nick Desaulniers wrote:
> On Mon, Jun 19, 2023 at 2:17 PM Yonghong Song <yhs@xxxxxxxx> 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.