Re: [PATCH v7 2/2] Kbuild: implement support for DWARF v5

[Sedat Dilek <sedat.dilek@gmail.com>]

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=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@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       += -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 -

[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
>