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Reply-To: sedat.dilek@gmail.com
From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Sun, 31 Jan 2021 01:39:52 +0100
Message-ID: <CA+icZUWQZKFy22PqMKmskPz8tRMTrAYDuevWZGVVC+9RPmMxZw@mail.gmail.com>
Subject: Re: [PATCH v7 2/2] Kbuild: implement support for DWARF v5
To:     =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= <maskray@google.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@googlegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jakub Jelinek <jakub@redhat.com>,
        Caroline Tice <cmtice@google.com>,
        Nick Clifton <nickc@redhat.com>, Yonghong Song <yhs@fb.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Arvind Sankar <nivedita@alum.mit.edu>
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On Sun, Jan 31, 2021 at 1:37 AM F=C4=81ng-ru=C3=AC S=C3=B2ng <maskray@googl=
e.com> wrote:
>
> On Sat, Jan 30, 2021 at 3:10 PM Sedat Dilek <sedat.dilek@gmail.com> wrote=
:
> >
> > On Sat, Jan 30, 2021 at 1:44 AM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > DWARF v5 is the latest standard of the DWARF debug info format.
> > >
> > > Feature detection of DWARF5 is onerous, especially given that we've
> > > removed $(AS), so we must query $(CC) for DWARF5 assembler directive
> > > support.
> > >
> > > The DWARF version of a binary can be validated with:
> > > $ llvm-dwarfdump vmlinux | head -n 4 | grep version
> > > or
> > > $ readelf --debug-dump=3Dinfo vmlinux 2>/dev/null | grep Version
> > >
> > > DWARF5 wins significantly in terms of size when mixed with compressio=
n
> > > (CONFIG_DEBUG_INFO_COMPRESSED).
> > >
> > > 363M    vmlinux.clang12.dwarf5.compressed
> > > 434M    vmlinux.clang12.dwarf4.compressed
> > > 439M    vmlinux.clang12.dwarf2.compressed
> > > 457M    vmlinux.clang12.dwarf5
> > > 536M    vmlinux.clang12.dwarf4
> > > 548M    vmlinux.clang12.dwarf2
> > >
> > > 515M    vmlinux.gcc10.2.dwarf5.compressed
> > > 599M    vmlinux.gcc10.2.dwarf4.compressed
> > > 624M    vmlinux.gcc10.2.dwarf2.compressed
> > > 630M    vmlinux.gcc10.2.dwarf5
> > > 765M    vmlinux.gcc10.2.dwarf4
> > > 809M    vmlinux.gcc10.2.dwarf2
> > >
> > > Though the quality of debug info is harder to quantify; size is not a
> > > proxy for quality.
> > >
> > > Jakub notes:
> > >   All [GCC] 5.1 - 6.x did was start accepting -gdwarf-5 as experiment=
al
> > >   option that enabled some small DWARF subset (initially only a few
> > >   DW_LANG_* codes newly added to DWARF5 drafts).  Only GCC 7 (release=
d
> > >   after DWARF 5 has been finalized) started emitting DWARF5 section
> > >   headers and got most of the DWARF5 changes in...
> > >
> > > Version check GCC so that we don't need to worry about the difference=
 in
> > > command line args between GNU readelf and llvm-readelf/llvm-dwarfdump=
 to
> > > validate the DWARF Version in the assembler feature detection script.
> > >
> > > GNU `as` only recently gained support for specifying -gdwarf-5, so wh=
en
> > > compiling with Clang but without Clang's integrated assembler
> > > (LLVM_IAS=3D1 is not set), explicitly add -Wa,-gdwarf-5 to DEBUG_CFLA=
GS.
> > >
> > > Disabled for now if CONFIG_DEBUG_INFO_BTF is set; pahole doesn't yet
> > > recognize the new additions to the DWARF debug info. Thanks to Sedat =
for
> > > the report.
> > >
> > > Link: http://www.dwarfstd.org/doc/DWARF5.pdf
> > > Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
> > > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
> > > Suggested-by: Caroline Tice <cmtice@google.com>
> > > Suggested-by: Fangrui Song <maskray@google.com>
> > > Suggested-by: Jakub Jelinek <jakub@redhat.com>
> > > Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> > > Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > > ---
> > >  Makefile                          |  1 +
> > >  include/asm-generic/vmlinux.lds.h |  7 ++++++-
> > >  lib/Kconfig.debug                 | 18 ++++++++++++++++++
> > >  scripts/test_dwarf5_support.sh    |  8 ++++++++
> > >  4 files changed, 33 insertions(+), 1 deletion(-)
> > >  create mode 100755 scripts/test_dwarf5_support.sh
> > >
> > > diff --git a/Makefile b/Makefile
> > > index d2b4980807e0..5387a6f2f62d 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -831,6 +831,7 @@ KBUILD_AFLAGS       +=3D -Wa,-gdwarf-2
> > >  endif
> > >
> > >  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) :=3D 4
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) :=3D 5
> > >  DEBUG_CFLAGS   +=3D -gdwarf-$(dwarf-version-y)
> > >
> > >  ifdef CONFIG_DEBUG_INFO_REDUCED
> > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/=
vmlinux.lds.h
> > > index 34b7e0d2346c..1e7cde4bd3f9 100644
> > > --- a/include/asm-generic/vmlinux.lds.h
> > > +++ b/include/asm-generic/vmlinux.lds.h
> > > @@ -842,8 +842,13 @@
> > >                 /* DWARF 4 */                                        =
   \
> > >                 .debug_types    0 : { *(.debug_types) }              =
   \
> > >                 /* DWARF 5 */                                        =
   \
> > > +               .debug_addr     0 : { *(.debug_addr) }               =
   \
> > > +               .debug_line_str 0 : { *(.debug_line_str) }           =
   \
> > > +               .debug_loclists 0 : { *(.debug_loclists) }           =
   \
> > >                 .debug_macro    0 : { *(.debug_macro) }              =
   \
> > > -               .debug_addr     0 : { *(.debug_addr) }
> > > +               .debug_names    0 : { *(.debug_names) }              =
   \
> > > +               .debug_rnglists 0 : { *(.debug_rnglists) }           =
   \
> > > +               .debug_str_offsets      0 : { *(.debug_str_offsets) }
> > >
> >
> > I just looked at binutils 2.36 in the Debian/experimental repositories.
> >
> > [1] says:
> >
> > + PR ld/27230
> > =EF=BF=BC+ * scripttempl/DWARF.sc: Add DWARF-5 .debug_* sections.
> >
> > ...
> >
> > -  /* DWARF Extension.  */
> > =EF=BF=BC-  .debug_macro    0 : { *(.debug_macro) }
> > +  /* DWARF 5.  */
> >    .debug_addr     0 : { *(.debug_addr) }
> > +  .debug_line_str 0 : { *(.debug_line_str) }
> > +  .debug_loclists 0 : { *(.debug_loclists) }
> > +  .debug_macro    0 : { *(.debug_macro) }
> > +  .debug_names    0 : { *(.debug_names) }
> > +  .debug_rnglists 0 : { *(.debug_rnglists) }
> > +  .debug_str_offsets 0 : { *(.debug_str_offsets) }
> > =EF=BF=BC+  .debug_sup      0 : { *(.debug_sup) }
> >
> > The list of DWARF-5 .debug_* sections is alphabetically sorted.
> > AFAICS .debug_sup section is missing?
> >
> > - Sedat -
>
> No compiler produces .debug_sup section. It could be from some
> post-processing tool which is unrelated to the linker.
> Omitting it is fine.
>

Thanks for the clarification, Fangrui.

- Sedat -

> > [1] https://salsa.debian.org/toolchain-team/binutils/-/commit/f58f33081=
035672b01a04326a9c8daadbd09a430
> >
> > >  /* Stabs debugging sections. */
> > >  #define STABS_DEBUG                                                 =
   \
> > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > > index 94c1a7ed6306..ad6f78989d4f 100644
> > > --- a/lib/Kconfig.debug
> > > +++ b/lib/Kconfig.debug
> > > @@ -268,6 +268,24 @@ config DEBUG_INFO_DWARF4
> > >           It makes the debug information larger, but it significantly
> > >           improves the success of resolving variables in gdb on optim=
ized code.
> > >
> > > +config DEBUG_INFO_DWARF5
> > > +       bool "Generate DWARF Version 5 debuginfo"
> > > +       depends on GCC_VERSION >=3D 50000 || CC_IS_CLANG
> > > +       depends on CC_IS_GCC || $(success,$(srctree)/scripts/test_dwa=
rf5_support.sh $(CC) $(CLANG_FLAGS))
> > > +       depends on !DEBUG_INFO_BTF
> > > +       help
> > > +         Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc=
 5.0+ (gcc
> > > +         5.0+ accepts the -gdwarf-5 flag but only had partial suppor=
t for some
> > > +         draft features until 7.0), and gdb 8.0+.
> > > +
> > > +         Changes to the structure of debug info in Version 5 allow f=
or around
> > > +         15-18% savings in resulting image and debug info section si=
zes as
> > > +         compared to DWARF Version 4. DWARF Version 5 standardizes p=
revious
> > > +         extensions such as accelerators for symbol indexing and the=
 format
> > > +         for fission (.dwo/.dwp) files. Users may not want to select=
 this
> > > +         config if they rely on tooling that has not yet been update=
d to
> > > +         support DWARF Version 5.
> > > +
> > >  endchoice # "DWARF version"
> > >
> > >  config DEBUG_INFO_BTF
> > > diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_sup=
port.sh
> > > new file mode 100755
> > > index 000000000000..c46e2456b47a
> > > --- /dev/null
> > > +++ b/scripts/test_dwarf5_support.sh
> > > @@ -0,0 +1,8 @@
> > > +#!/bin/sh
> > > +# SPDX-License-Identifier: GPL-2.0
> > > +
> > > +# Test that the assembler doesn't need -Wa,-gdwarf-5 when presented =
with DWARF
> > > +# v5 input, such as `.file 0` and `md5 0x00`. Should be fixed in GNU=
 binutils
> > > +# 2.35.2. https://sourceware.org/bugzilla/show_bug.cgi?id=3D25611
> > > +echo '.file 0 "filename" md5 0x7a0b65214090b6693bd1dc24dd248245' | \
> > > +  $* -gdwarf-5 -Wno-unused-command-line-argument -c -x assembler -o =
/dev/null -
> > > --
> > > 2.30.0.365.g02bc693789-goog
> > >