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

[Yonghong Song]

On 6/20/23 8:07 AM, Nick Desaulniers wrote:
> On Tue, Jun 20, 2023 at 10:53 AM Yonghong Song <yhs@xxxxxxxx> 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.