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Date: Sat, 24 Sep 2022 11:11:04 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Michal Marek <michal.lkml@...kovi.net>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
clang-built-linux <llvm@...ts.linux.dev>,
Bill Wendling <morbo@...gle.com>,
Greg Thelen <gthelen@...gle.com>,
Alexey Alexandrov <aalexand@...gle.com>,
Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v4] Makefile.debug: re-enable debug info for .S files
On Tue, Sep 20, 2022 at 2:45 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> Alexey reported that the fraction of unknown filename instances in
> kallsyms grew from ~0.3% to ~10% recently; Bill and Greg tracked it down
> to assembler defined symbols, which regressed as a result of:
>
> commit b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
>
> In that commit, I allude to restoring debug info for assembler defined
> symbols in a follow up patch, but it seems I forgot to do so in
>
> commit a66049e2cf0e ("Kbuild: make DWARF version a choice")
>
> Link: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
> Fixes: b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
> Reported-by: Alexey Alexandrov <aalexand@...gle.com>
> Reported-by: Bill Wendling <morbo@...gle.com>
> Reported-by: Greg Thelen <gthelen@...gle.com>
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Changes v3 -> v4:
> * Break out into its own patch.
> * Move addition of debug-flags-y to DEBUG_CFLAGS and KBUILD_AFLAGS up
> grouping the -gdwarf-* handling together.
>
> lib/Kconfig.debug | 4 +++-
> scripts/Makefile.debug | 18 +++++++++---------
> 2 files changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index bcbe60d6c80c..d3e5f36bb01e 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -264,8 +264,10 @@ config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> config DEBUG_INFO_DWARF4
> bool "Generate DWARF Version 4 debuginfo"
> select DEBUG_INFO
> + depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)))
> help
> - Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
> + Generate DWARF v4 debug info. This requires gcc 4.5+, binutils 2.35.2
> + if using clang without clang's integrated assembler, and gdb 7.0+.
>
> If you have consumers of DWARF debug info that are not ready for
> newer revisions of DWARF, you may wish to choose this or have your
> diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> index 26d6a9d97a20..d6aecd78b942 100644
> --- a/scripts/Makefile.debug
> +++ b/scripts/Makefile.debug
> @@ -4,15 +4,15 @@ ifdef CONFIG_DEBUG_INFO_SPLIT
> DEBUG_CFLAGS += -gsplit-dwarf
> endif
>
> -ifndef CONFIG_AS_IS_LLVM
> -KBUILD_AFLAGS += -Wa,-gdwarf-2
> -endif
> -
> -ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> -dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> -dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
> -DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
> +debug-flags-$(CONFIG_DEBUG_INFO_DWARF4) += -gdwarf-4
> +debug-flags-$(CONFIG_DEBUG_INFO_DWARF5) += -gdwarf-5
> +ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_AS_IS_GNU),yy)
> +# Clang does not pass -g or -gdwarf-* option down to GAS.
This patch still misses the debug info for *.S files
for the combination of LLVM_IAS=0 and
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
because, as the comment says, Clang does not pass -g down to GAS.
With "[v4] Makefile.debug: set -g unconditional on CONFIG_DEBUG_INFO_SPLIT"
and this one applied,
$ grep CONFIG_DEBUG_INFO_DWARF .config
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
# CONFIG_DEBUG_INFO_DWARF4 is not set
# CONFIG_DEBUG_INFO_DWARF5 is not set
$ make LLVM=1 LLVM_IAS=0 arch/x86/kernel/irqflags.o
SYNC include/config/auto.conf.cmd
SYSHDR arch/x86/include/generated/asm/unistd_32_ia32.h
SYSHDR arch/x86/include/generated/asm/unistd_64_x32.h
SYSTBL arch/x86/include/generated/asm/syscalls_64.h
HOSTCC arch/x86/tools/relocs_32.o
[snip]
AS arch/x86/kernel/irqflags.o
$ objdump -h arch/x86/kernel/irqflags.o | grep debug
$
I think the following fix-up is needed on top.
diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
index d6aecd78b942..8cf1cb22dd93 100644
--- a/scripts/Makefile.debug
+++ b/scripts/Makefile.debug
@@ -1,4 +1,5 @@
-DEBUG_CFLAGS := -g
+DEBUG_CFLAGS :=
+debug-flags-y := -g
ifdef CONFIG_DEBUG_INFO_SPLIT
DEBUG_CFLAGS += -gsplit-dwarf
Then, I can see the debug sections.
$ make LLVM=1 LLVM_IAS=0 arch/x86/kernel/irqflags.o
CALL scripts/checksyscalls.sh
DESCEND objtool
AS arch/x86/kernel/irqflags.o
$ objdump -h arch/x86/kernel/irqflags.o | grep debug
6 .debug_line 00000050 0000000000000000 0000000000000000 0000008f 2**0
7 .debug_info 0000002e 0000000000000000 0000000000000000 000000f8 2**0
8 .debug_abbrev 00000014 0000000000000000 0000000000000000 000001d0 2**0
9 .debug_aranges 00000030 0000000000000000 0000000000000000 000001f0 2**4
10 .debug_str 0000004d 0000000000000000 0000000000000000 00000250 2**0
If you agree, I can locally fix it up as such.
--
Best Regards
Masahiro Yamada
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