Date:   Fri, 29 Jan 2021 14:47:50 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        linux-kbuild@...r.kernel.org, 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 v6 2/2] Kbuild: implement support for DWARF v5

On 2021-01-29, Nick Desaulniers 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                          | 12 ++++++++++++
> include/asm-generic/vmlinux.lds.h |  6 +++++-
> lib/Kconfig.debug                 | 18 ++++++++++++++++++
> scripts/test_dwarf5_support.sh    |  8 ++++++++
> 4 files changed, 43 insertions(+), 1 deletion(-)
> create mode 100755 scripts/test_dwarf5_support.sh
>
>diff --git a/Makefile b/Makefile
>index 20141cd9319e..bed8b3b180b8 100644
>--- a/Makefile
>+++ b/Makefile
>@@ -832,8 +832,20 @@ endif
>
> dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
>+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
> DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
>
>+# If using clang without the integrated assembler, we need to explicitly tell
>+# GAS that we will be feeding it DWARF v5 assembler directives. Kconfig should
>+# detect whether the version of GAS supports DWARF v5.
>+ifdef CONFIG_CC_IS_CLANG
>+ifneq ($(LLVM_IAS),1)
>+ifeq ($(dwarf-version-y),5)
>+DEBUG_CFLAGS	+= -Wa,-gdwarf-5
>+endif
>+endif
>+endif
>+
> ifdef CONFIG_DEBUG_INFO_REDUCED
> DEBUG_CFLAGS	+= $(call cc-option, -femit-struct-debug-baseonly) \
> 		   $(call cc-option,-fno-var-tracking)
>diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>index 34b7e0d2346c..f8d5455cd87f 100644
>--- a/include/asm-generic/vmlinux.lds.h
>+++ b/include/asm-generic/vmlinux.lds.h
>@@ -843,7 +843,11 @@
> 		.debug_types	0 : { *(.debug_types) }			\
> 		/* DWARF 5 */						\
> 		.debug_macro	0 : { *(.debug_macro) }			\
>-		.debug_addr	0 : { *(.debug_addr) }
>+		.debug_addr	0 : { *(.debug_addr) }			\
>+		.debug_line_str	0 : { *(.debug_line_str) }		\
>+		.debug_loclists	0 : { *(.debug_loclists) }		\
>+		.debug_rnglists	0 : { *(.debug_rnglists) }		\
>+		.debug_str_offsets	0 : { *(.debug_str_offsets) }
>
> /* Stabs debugging sections. */
> #define STABS_DEBUG							\
>diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>index 1850728b23e6..09146b1af20d 100644
>--- a/lib/Kconfig.debug
>+++ b/lib/Kconfig.debug
>@@ -273,6 +273,24 @@ 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, 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..1a00484d0b2e
>--- /dev/null
>+++ b/scripts/test_dwarf5_support.sh
>@@ -0,0 +1,8 @@
>+#!/bin/sh
>+# SPDX-License-Identifier: GPL-2.0
>+
>+# Test that assembler accepts -gdwarf-5 and .file 0 directives, which were bugs
>+# in binutils < 2.35.
>+# https://sourceware.org/bugzilla/show_bug.cgi?id=25612
>+# https://sourceware.org/bugzilla/show_bug.cgi?id=25614
>+echo '.file 0 "filename"' | $* -gdwarf-5 -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
>-- 
>2.30.0.365.g02bc693789-goog
>

Since the other thread suggested that you keep just DWARF v4 and v5 and
drop DWARF v2, you'll need v7...

I suggest:

Change the -Wa,-gdwarf-2 logic to add either -Wa,-gdwarf-4 or
-Wa,-gdwarf-5 and drop the CONFIG_CC_IS_CLANG ifdef.
(CONFIG_CC_IS_CLANG is there because clang -Wa,-gdwarf-2 does not allow
DW_AT_ranges which is not in DWARF v2).

GNU as before 2020 did not support -gdwarf-[345]
(https://sourceware.org/bugzilla/show_bug.cgi?id=25612), so you can make
-Wa,-gdwarf-2 a DWARF v4 workaround for older GNU as.

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