Re: [PATCH v5] MIPS: Truncate link address into 32bit for 32bit kernel

From: Jiaxun Yang
Date: Thu Apr 23 2020 - 01:44:23 EST




ä 2020å4æ23æ GMT+08:00 äå8:10:12, "Maciej W. Rozycki" <macro@xxxxxxxxxxxxxx> åå:
>On Wed, 22 Apr 2020, Jiaxun Yang wrote:
>
>> Reviewed-by: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx>
>
> Hmm, that was for an earlier version of the patch, and reviews obviously
>do not automatically carry over to subsequent versions, as it cannot be
>claimed that they are as good in the reviewer's eyes as the actual version
>reviewed was.
>
>> diff --git a/arch/mips/Makefile b/arch/mips/Makefile
>> index e1c44aed8156..68c0f22fefc0 100644
>> --- a/arch/mips/Makefile
>> +++ b/arch/mips/Makefile
>> @@ -288,12 +288,23 @@ ifdef CONFIG_64BIT
>> endif
>> endif
>>
>> +# When linking a 32-bit executable the LLVM linker cannot cope with a
>> +# 32-bit load address that has been sign-extended to 64 bits. Simply
>> +# remove the upper 32 bits then, as it is safe to do so with other
>> +# linkers.
>> +ifdef CONFIG_64BIT
>> + load-ld = $(load-y)
>> +else
>> + load-ld = $(subst 0xffffffff,0x,$(load-y))
>> +endif
>> +
>> KBUILD_AFLAGS += $(cflags-y)
>> KBUILD_CFLAGS += $(cflags-y)
>> -KBUILD_CPPFLAGS += -DVMLINUX_LOAD_ADDRESS=$(load-y)
>> +KBUILD_CPPFLAGS += -DVMLINUX_LOAD_ADDRESS=$(load-y) -DVMLINUX_LINK_ADDRESS=$(load-ld)
>> KBUILD_CPPFLAGS += -DDATAOFFSET=$(if $(dataoffset-y),$(dataoffset-y),0)
>>
>> bootvars-y = VMLINUX_LOAD_ADDRESS=$(load-y) \
>> + VMLINUX_LINK_ADDRESS=$(load-ld) \
>> VMLINUX_ENTRY_ADDRESS=$(entry-y) \
>> PLATFORM="$(platform-y)" \
>> ITS_INPUTS="$(its-y)"
>
> Hmm, to be honest I find the nomenclature confusing: VMLINUX_LOAD_ADDRESS
>and VMLINUX_LINK_ADDRESS sound like synonyms to me and also look very
>similar, so I expect people will be confused and scratch their heads in
>the future. Due to the obscurity of the problem I think there is little
>room for manoeuvre here really, but how about using LINKER_LOAD_ADDRESS
>for the new variable?
>
> Alternatively, have you made any attempt to verify if actually replacing
>the setting for VMLINUX_LOAD_ADDRESS would be safe? Glancing over its use
>there do not appear to be many places.

Limited experiments showed it should be fine...

But MIPS kernel has some design I'm not really familiar with like SYM32 for
64-bit kernel and special address space design for Trap-and-emul KVM.

I'm not 100 sure it's safe so I didn't do so.

>
> Maciej

--
Jiaxun Yang