Re: [PATCH v2 3/4] Kbuild: make DWARF version a choice

From: Masahiro Yamada <masahiroy@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Jakub Jelinek <jakub@redhat.com>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-toolchains@vger.kernel.org,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Fangrui Song <maskray@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Dmitry Golovin <dima@golovin.in>,
	Alistair Delva <adelva@google.com>
Subject: Re: [PATCH v2 3/4] Kbuild: make DWARF version a choice
Date: Tue, 1 Dec 2020 13:17:42 +0900	[thread overview]
Message-ID: <CAK7LNATMjv84JCNX7ZDNkhA13he8SZYHES1i5k5EZzbD1iKqfA@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdmPeOEA4dfODCKLE4A_M-SF5RBVFEf-NuiTkUTXAbh-5w@mail.gmail.com>

On Tue, Dec 1, 2020 at 5:45 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> On Mon, Nov 30, 2020 at 10:05 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > On Wed, Nov 4, 2020 at 9:53 AM 'Nick Desaulniers' via Clang Built
> > Linux <clang-built-linux@googlegroups.com> wrote:
> > >
> > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice. Adds an
> > > explicit CONFIG_DEBUG_INFO_DWARF2, which is the default. Does so in a
> > > way that's forward compatible with existing configs, and makes adding
> > > future versions more straightforward.
> > >
> > > Suggested-by: Fangrui Song <maskray@google.com>
> > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > > ---
> > >  Makefile          | 14 ++++++++------
> > >  lib/Kconfig.debug | 19 +++++++++++++++----
> > >  2 files changed, 23 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index 75b1a3dcbf30..e23786a4c1c7 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -826,12 +826,14 @@ else
> > >  DEBUG_CFLAGS   += -g
> > >  endif
> > >
> > > -ifndef LLVM_IAS
> > > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > > -endif
> > > -
> > > -ifdef CONFIG_DEBUG_INFO_DWARF4
> > > -DEBUG_CFLAGS   += -gdwarf-4
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > > +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
> > > +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
> > > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > > +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > > +DEBUG_CFLAGS   += $(dwarf-aflag)
> >
> > This changes the behavior.
> >
> > For the Dwarf-2 case,
> >
> > Previously, -gdwarf-2 was passed to $(CC),
> > so the debug info was generated by gcc.
> >
> > Now, -Wa,-gdwarf-2 is passed to $(CC).
> > -gdwarf-2 is handled by GNU as.
> > So, the source info points to /tmp/<hash>.s
> > instead of the original .c file.
> >
> >
> >
> > Handling the Dwarf capability is very complicated.
> >
> > Are you still working for v3?
>
> Yes, I plan to revisit the series based on all of the feedback thus
> far.  Lately I'm focused on enabling LLVM_IAS=1 for Android; but I
> would like to see this land so that the Linux kernel may provide
> coverage and feedback to the toolchain developers for DWARF v5 (as
> well as reduced binary image sizes).  Maybe later this week I'll have
> time to revisit.
> --
> Thanks,
> ~Nick Desaulniers
>
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In my understanding, the complexity comes from the fact
we are mixing up the $(CC) capability and $(AS) capability.

They are orthogonal if I understand correctly.

When building *.c files, the .debug* sections are generated by
gcc (or clang), and embedded into the intermediate *.s files.
The assembler (GAS or clang's IAS) simply transforms it
into byte stream in *.o. So we do not care about the assembler capability.

When building *.S files, the .debug* sections are generated by
the assembler. Here, the assembler capability is important.
Unless we use binutils 2.35+ or clang IAS,
DWARF v2 is the only possible choice.

So, we need two separate choices to handle this properly, I think.

The following is the rough sketch.

# The value is 2, 3, 4, or 5 depending on the assembler in use.
# Unfortunately, we cannot check this by $(cc-option, -Wa,-gdwarf-4)
# because GAS <= 2.34 accepts any -gdwarf-<N>.
# readelf --debug-dump=info and grep or something?
config AS_SUPPORTS_DWARF_VERSION
        int $(shell scripts/as_dwarf_support.sh)

choice
            "DWARF version for C code debugging"

config CC_DEBUG_INFO_DWARF2
            bool "..."

config CC_DEBUG_INFO_DWARF4
            bool "..."

config CC_DEBUG_INFO_DWARF5
            bool "..."
            depends on GCC_VERSION >= 700000 || CC_IS_CLANG
            depends on AS_SUPPORTS_DWARF_VERSION >= 5
            help
              gcc7+ or clang supports this.
              Unfortunately, we also need to check assembler capability
              because GAS <= 2.34 do not understand ".file 0"

endchoice

choice
            "DWARF version for assembly code debugging"

config AS_DEBUG_INFO_DWARF2
            bool "..."

config AS_DEBUG_INFO_DWARF4
            bool "..."
            depends on AS_SUPPORTS_DWARF_VERSION >= 4

config AS_DEBUG_INFO_DWARF5
            bool "..."
            depends on AS_SUPPORTS_DWARF_VERSION >= 5

endchoice

-- 
Best Regards
Masahiro Yamada