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Message-ID: <CAFP8O3LaWwa5_Y49fr+PDn6y7NefUDRt=KuVuuo0Gf+kZNkLzA@mail.gmail.com>
Date: Sat, 30 Jan 2021 16:37:34 -0800
From: Fāng-ruì Sòng <maskray@...gle.com>
To: Sedat Dilek <sedat.dilek@...il.com>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>,
Masahiro Yamada <masahiroy@...nel.org>,
Nathan Chancellor <natechancellor@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
linux-arch <linux-arch@...r.kernel.org>,
Jakub Jelinek <jakub@...hat.com>,
Caroline Tice <cmtice@...gle.com>,
Nick Clifton <nickc@...hat.com>, Yonghong Song <yhs@...com>,
Jiri Olsa <jolsa@...nel.org>,
Andrii Nakryiko <andrii@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v7 2/2] Kbuild: implement support for DWARF v5
On Sat, Jan 30, 2021 at 3:10 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> On Sat, Jan 30, 2021 at 1:44 AM Nick Desaulniers
> <ndesaulniers@...gle.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=info vmlinux 2>/dev/null | grep Version
> >
> > DWARF5 wins significantly in terms of size when mixed with compression
> > (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 experimental
> > option that enabled some small DWARF subset (initially only a few
> > DW_LANG_* codes newly added to DWARF5 drafts). Only GCC 7 (released
> > 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 when
> > compiling with Clang but without Clang's integrated assembler
> > (LLVM_IAS=1 is not set), explicitly add -Wa,-gdwarf-5 to DEBUG_CFLAGS.
> >
> > 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@...il.com>
> > Suggested-by: Arvind Sankar <nivedita@...m.mit.edu>
> > Suggested-by: Caroline Tice <cmtice@...gle.com>
> > Suggested-by: Fangrui Song <maskray@...gle.com>
> > Suggested-by: Jakub Jelinek <jakub@...hat.com>
> > Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> > Suggested-by: Nathan Chancellor <natechancellor@...il.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.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 += -Wa,-gdwarf-2
> > endif
> >
> > dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
> > DEBUG_CFLAGS += -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
> + * scripttempl/DWARF.sc: Add DWARF-5 .debug_* sections.
>
> ...
>
> - /* DWARF Extension. */
> - .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) }
> + .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.
> [1] https://salsa.debian.org/toolchain-team/binutils/-/commit/f58f33081035672b01a04326a9c8daadbd09a430
>
> > /* 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 optimized code.
> >
> > +config DEBUG_INFO_DWARF5
> > + bool "Generate DWARF Version 5 debuginfo"
> > + depends on GCC_VERSION >= 50000 || CC_IS_CLANG
> > + depends on CC_IS_GCC || $(success,$(srctree)/scripts/test_dwarf5_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 support for some
> > + draft features until 7.0), and gdb 8.0+.
> > +
> > + Changes to the structure of debug info in Version 5 allow for around
> > + 15-18% savings in resulting image and debug info section sizes as
> > + compared to DWARF Version 4. DWARF Version 5 standardizes previous
> > + 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 updated to
> > + support DWARF Version 5.
> > +
> > endchoice # "DWARF version"
> >
> > config DEBUG_INFO_BTF
> > diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.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=25611
> > +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
> >
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