linux-kernel - Re: [PATCH v2 2/4] Kbuild: do not emit debug info for assembly with LLVM

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Date:   Wed, 4 Nov 2020 23:26:53 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Jakub Jelinek <jakub@...hat.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Alistair Delva <adelva@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/4] Kbuild: do not emit debug info for assembly with
 LLVM_IAS=1


On 2020-11-04, Nathan Chancellor wrote:
>On Tue, Nov 03, 2020 at 04:53:41PM -0800, Nick Desaulniers wrote:
>> Clang's integrated assembler produces the warning for assembly files:
>>
>> warning: DWARF2 only supports one section per compilation unit
>>
>> If -Wa,-gdwarf-* is unspecified, then debug info is not emitted.  This
>
>Is this something that should be called out somewhere? If I understand
>this correctly, LLVM_IAS=1 + CONFIG_DEBUG_INFO=y won't work? Maybe this
>should be handled in Kconfig?
>
>> will be re-enabled for new DWARF versions in a follow up patch.
>>
>> Enables defconfig+CONFIG_DEBUG_INFO to build cleanly with
>> LLVM=1 LLVM_IAS=1 for x86_64 and arm64.
>>
>> Cc: <stable@...r.kernel.org>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/716
>> Reported-by: Nathan Chancellor <natechancellor@...il.com>
>> Suggested-by: Dmitry Golovin <dima@...ovin.in>
>
>If you happen to respin, Dmitry deserves a Reported-by tag too :)
>
>> Suggested-by: Sedat Dilek <sedat.dilek@...il.com>
>> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
>Regardless of the other two comments, this is fine as is as a fix for
>stable to unblock Android + CrOS since we have been running something
>similar to it in CI:
>
>Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
>
>> ---
>>  Makefile | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/Makefile b/Makefile
>> index f353886dbf44..75b1a3dcbf30 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -826,7 +826,9 @@ else
>>  DEBUG_CFLAGS	+= -g
>>  endif
>>
>> +ifndef LLVM_IAS
>
>Nit: this should probably match the existing LLVM_IAS check
>
>ifneq ($(LLVM_IAS),1)
>
>>  KBUILD_AFLAGS	+= -Wa,-gdwarf-2
>> +endif
>>
>>  ifdef CONFIG_DEBUG_INFO_DWARF4
>>  DEBUG_CFLAGS	+= -gdwarf-4
>> --
>> 2.29.1.341.ge80a0c044ae-goog
>>

The root cause is that DWARF v2 has no DW_AT_ranges, so it cannot
represent non-contiguous address ranges. It seems that GNU as -gdwarf-3
emits DW_AT_ranges as well and emits an entry for a non-executable section.
In any case, the option is of very low value, at least for LLVM.


Reviewed-by: Fangrui Song <maskray@...gle.com>