linux-kernel - Re: [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files

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Date:   Tue, 6 Sep 2022 21:27:07 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        X86 ML <x86@...nel.org>,
        Dmitrii Bundin <dmitrii.bundin.a@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Alexey Alexandrov <aalexand@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files

On Mon, Sep 5, 2022 at 12:50 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Thu, Sep 1, 2022 at 3:44 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> >
> > Alexey reported that the fraction of unknown filename instances in
> > kallsyms grew from ~0.3% to ~10% recently; Bill and Greg tracked it down
> > to assembler defined symbols, which regressed as a result of:
> >
> > commit b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
> >
> > In that commit, I allude to restoring debug info for assembler defined
> > symbols in a follow up patch, but it seems I forgot to do so in
> >
> > commit a66049e2cf0e ("Kbuild: make DWARF version a choice")
> >
> > This patch does a few things:
> > 1. Add -g to KBUILD_AFLAGS. This will instruct the compiler to instruct
> >    the assembler to emit debug info. But this can cause an issue for
> >    folks using a newer compiler but older assembler, because the
> >    implicit default DWARF version changed from v4 to v5 in gcc-11 and
> >    clang-14.
>
>
>
> What kind of bad things happen for "KBUILD_AFLAGS += -g"?
>
>
> I think 'gcc -g -c -o foo.o foo.S' will invoke 'as --gdwarf-2' as the backend
> if gcc is configured to work with old binutils.

That's fine for CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT I think?

What other problems were you envisioning?

>
>
>
>
> > diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> > index 9f39b0130551..46e88f0ca998 100644
> > --- a/scripts/Makefile.debug
> > +++ b/scripts/Makefile.debug
> > @@ -4,18 +4,32 @@ ifdef CONFIG_DEBUG_INFO_SPLIT
> >  DEBUG_CFLAGS   += -gsplit-dwarf
> >  else
> >  DEBUG_CFLAGS   += -g
> > +KBUILD_AFLAGS  += -g
> >  endif
> >
> > -ifndef CONFIG_AS_IS_LLVM
> > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > +ifdef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> > +# gcc-11+, clang-14+
> > +ifeq ($(call cc-min-version, 110000, 140000),y)
> > +dwarf-version-y := 5
> > +else
> > +dwarf-version-y := 4
>
>
>
> If you explicitly specify the DWARF version
> for CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT,
> what is the point of CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT?
>
>
> When CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y,
> I believe the right thing to do is to pass only -g,
> and let the tool do whatever it thinks is appropriate.

Ok, sure, I will revise.

>
>
>
>
>
>
> >  endif
> > -
> > -ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> > +else # !CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> >  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> >  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
> >  DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
> >  endif
> >
> > +# Binutils 2.35+ (or clang) required for -gdwarf-{4|5}.
> > +# https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
> > +ifneq ($(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y)),)
>
>
>
> When is this as-option supposed to fail?
>
>
> Binutils <= 2.34 always accepts whatever -gdwarf-* option.
> Surprisingly or not, it accepts -gdwarf-6, -gdwarf-7, ...
>
> No matter what DWARF version you specify, GAS silently downgrades
> it to DWARF-2.
>
>
> masahiro@zoe:~/tools/binutils-2.34/bin$ ./as --version | head -n 1
> GNU assembler (GNU Binutils) 2.34
> masahiro@zoe:~/tools/binutils-2.34/bin$ cat /dev/null | ./as -gdwarf-5
> -o /dev/null -
> masahiro@zoe:~/tools/binutils-2.34/bin$ echo $?
> 0
> masahiro@zoe:~/tools/binutils-2.34/bin$ cat /dev/null | ./as
> -gdwarf-100 -o /dev/null -
> masahiro@zoe:~/tools/binutils-2.34/bin$ echo $?
> 0

ah, right. Maybe an explicit version check is necessary then.

>
>
>
>
> Overall, I am not convinced with this patch.
>
>
>
> Please see the attached patch.
> Is there any problem with writing this more simply?


Thanks for the inspiration, I will use that as an inspiration/base for
a new patch.
-- 
Thanks,
~Nick Desaulniers