linux-kernel - Re: [PATCH v2] MIPS: Truncate load-y into 32bit for 32bit kernel

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Date:   Tue, 7 Apr 2020 11:00:23 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Jiaxun Yang <jiaxun.yang@...goat.com>, linux-mips@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] MIPS: Truncate load-y into 32bit for 32bit kernel

On 2020-04-07, Nick Desaulniers wrote:
>On Tue, Apr 7, 2020 at 1:07 AM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
>>
>> LLD failed to link vmlinux with 64bit load address for 32bit ELF
>> while bfd will strip 64bit address into 32bit silently.
>> To fix LLD build, we should truncate load address provided by platform
>> into 32bit for 32bit kernel.
>>
>> Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
>> Reviewed-by: Fangrui Song <maskray@...gle.com>
>> Tested-by: Nathan Chancellor <natechancellor@...il.com>
>>
>> --
>> V2: Take MaskRay's shell magic.
>
>V2 is way too clever, V1 was much more readable.

This is difficult:/
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04
The POSIX shell is only required to do signed long integer arithmetic.
"signed long" can be 32-bit.

awk may not provide precision more than a double ("... decimal-floating-constant token as specified
by the ISO C standard")

   % gawk 'BEGIN {printf("%x", (0xffffffff80101234 % 0x100000000))}' /dev/null
   80101000

>Can this tag be added to the commit to help us track when and where it lands?
>Link: https://github.com/ClangBuiltLinux/linux/issues/786

And this tag for GNU ld enhancement:

Link: https://sourceware.org/bugzilla/show_bug.cgi?id=25784

>> ---
>>  arch/mips/Makefile | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/mips/Makefile b/arch/mips/Makefile
>> index e1c44aed8156..f8fd3c39fb55 100644
>> --- a/arch/mips/Makefile
>> +++ b/arch/mips/Makefile
>> @@ -286,6 +286,9 @@ ifdef CONFIG_64BIT
>>        $(error CONFIG_CPU_DADDI_WORKAROUNDS unsupported without -msym32)
>>      endif
>>    endif
>> +else
>> +       # Truncate address into 32-bit
>> +       load-y := 0x$(shell echo "$(load-y)" | rev | head -c 8 | rev)
>>  endif
>>
>>  KBUILD_AFLAGS  += $(cflags-y)
>> --
>
>-- 
>Thanks,
>~Nick Desaulniers