Re: [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files

From: Masahiro Yamada <masahiroy@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Michal Marek <michal.lkml@markovi.net>,
	Nathan Chancellor <nathan@kernel.org>, Tom Rix <trix@redhat.com>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	clang-built-linux <llvm@lists.linux.dev>, X86 ML <x86@kernel.org>,
	Dmitrii Bundin <dmitrii.bundin.a@gmail.com>,
	Fangrui Song <maskray@google.com>,
	Alexey Alexandrov <aalexand@google.com>,
	Bill Wendling <morbo@google.com>,
	Greg Thelen <gthelen@google.com>
Subject: Re: [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files
Date: Mon, 5 Sep 2022 16:49:09 +0900	[thread overview]
Message-ID: <CAK7LNATWDH01=ZKLnsxc0vcib1zGDbEq8jLQwhWP7HkkmSb_Mw@mail.gmail.com> (raw)
In-Reply-To: <20220831184408.2778264-5-ndesaulniers@google.com>

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On Thu, Sep 1, 2022 at 3:44 AM Nick Desaulniers <ndesaulniers@google.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")
>
> This patch does a few things:
> 1. Add -g to KBUILD_AFLAGS. This will instruct the compiler to instruct
>    the assembler to emit debug info. But this can cause an issue for
>    folks using a newer compiler but older assembler, because the
>    implicit default DWARF version changed from v4 to v5 in gcc-11 and
>    clang-14.



What kind of bad things happen for "KBUILD_AFLAGS += -g"?


I think 'gcc -g -c -o foo.o foo.S' will invoke 'as --gdwarf-2' as the backend
if gcc is configured to work with old binutils.




> diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> index 9f39b0130551..46e88f0ca998 100644
> --- a/scripts/Makefile.debug
> +++ b/scripts/Makefile.debug
> @@ -4,18 +4,32 @@ ifdef CONFIG_DEBUG_INFO_SPLIT
>  DEBUG_CFLAGS   += -gsplit-dwarf
>  else
>  DEBUG_CFLAGS   += -g
> +KBUILD_AFLAGS  += -g
>  endif
>
> -ifndef CONFIG_AS_IS_LLVM
> -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> +ifdef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> +# gcc-11+, clang-14+
> +ifeq ($(call cc-min-version, 110000, 140000),y)
> +dwarf-version-y := 5
> +else
> +dwarf-version-y := 4



If you explicitly specify the DWARF version
for CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT,
what is the point of CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT?


When CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y,
I believe the right thing to do is to pass only -g,
and let the tool do whatever it thinks is appropriate.






>  endif
> -
> -ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> +else # !CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
>  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
>  DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
>  endif
>
> +# Binutils 2.35+ (or clang) required for -gdwarf-{4|5}.
> +# https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
> +ifneq ($(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y)),)



When is this as-option supposed to fail?


Binutils <= 2.34 always accepts whatever -gdwarf-* option.
Surprisingly or not, it accepts -gdwarf-6, -gdwarf-7, ...

No matter what DWARF version you specify, GAS silently downgrades
it to DWARF-2.


masahiro@zoe:~/tools/binutils-2.34/bin$ ./as --version | head -n 1
GNU assembler (GNU Binutils) 2.34
masahiro@zoe:~/tools/binutils-2.34/bin$ cat /dev/null | ./as -gdwarf-5
-o /dev/null -
masahiro@zoe:~/tools/binutils-2.34/bin$ echo $?
0
masahiro@zoe:~/tools/binutils-2.34/bin$ cat /dev/null | ./as
-gdwarf-100 -o /dev/null -
masahiro@zoe:~/tools/binutils-2.34/bin$ echo $?
0




Overall, I am not convinced with this patch.



Please see the attached patch.
Is there any problem with writing this more simply?





> +KBUILD_AFLAGS  += -Wa,-gdwarf-$(dwarf-version-y)
> +else
> +ifndef CONFIG_AS_IS_LLVM
> +KBUILD_AFLAGS  += -Wa,-gdwarf-2
> +endif
> +endif
> +
>  ifdef CONFIG_DEBUG_INFO_REDUCED
>  DEBUG_CFLAGS   += -fno-var-tracking
>  ifdef CONFIG_CC_IS_GCC
> --
> 2.37.2.672.g94769d06f0-goog
>


--
Best Regards
Masahiro Yamada

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diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index bcbe60d6c80c..37aa19f0f7b0 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -264,6 +264,7 @@ 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 >= 23500)))
 	help
 	  Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
 
diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
index 9f39b0130551..c5ecb882a638 100644
--- a/scripts/Makefile.debug
+++ b/scripts/Makefile.debug
@@ -3,17 +3,20 @@ DEBUG_CFLAGS	:=
 ifdef CONFIG_DEBUG_INFO_SPLIT
 DEBUG_CFLAGS	+= -gsplit-dwarf
 else
-DEBUG_CFLAGS	+= -g
+debug-flags-y	+= -g
 endif
 
-ifndef CONFIG_AS_IS_LLVM
-KBUILD_AFLAGS	+= -Wa,-gdwarf-2
-endif
+debug-flags-$(CONFIG_DEBUG_INFO_DWARF4) += -gdwarf-4
+debug-flags-$(CONFIG_DEBUG_INFO_DWARF5) += -gdwarf-5
+
+DEBUG_CFLAGS	+= $(debug-flags-y)
 
-ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
-dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
-dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
-DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
+ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_AS_IS_GNU),yy)
+# Clang does not pass -g or -gdwarf-* option down to GAS.
+# Add -Wa, prefix to explicitly specify the flags.
+KBUILD_AFLAGS	+= $(addprefix -Wa$(comma), $(debug-flags-y))
+else
+KBUILD_AFLAGS	+= $(debug-flags-y)
 endif
 
 ifdef CONFIG_DEBUG_INFO_REDUCED
