Subject: [PATCH] arm64: Enable CONFIG_COMPAT also for 64k page size

From: [email-masked] (Christopher Covington)
Date: Wed, 18 Feb 2015 08:40:51 -0500

On 12/05/2014 06:05 AM, Catalin Marinas wrote:
> On Fri, Dec 05, 2014 at 10:39:40AM +0000, Arnd Bergmann wrote:
>> On Thursday 04 December 2014 15:48:50 Olof Johansson wrote:
>>> On Thu, Dec 4, 2014 at 3:41 PM, Alexander Graf <[email-masked]> wrote:
>>>> On 04.12.14 22:15, Olof Johansson wrote:
>>>>> On Thu, Dec 4, 2014 at 7:46 AM, Alexander Graf <[email-masked]> wrote:
>>>>>> With binutils 2.25 the default alignment for 32bit arm sections changed to
>>>>>> have everything 64k aligned. Armv7 binaries built with this binutils version
>>>>>> run successfully on an arm64 system.
>>>>>>
>>>>>> Since effectively there is now the chance to run armv7 code on arm64 even
>>>>>> with 64k page size, it doesn't make sense to block people from enabling
>>>>>> CONFIG_COMPAT on those configurations.
>>>>>>
>>>>>> Signed-off-by: Alexander Graf <[email-masked]>
>>>>>> ---
>>>>>>  arch/arm64/Kconfig | 1 -
>>>>>>  1 file changed, 1 deletion(-)
>>>>>>
>>>>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>>>>> index 9532f8d..3cf4f238 100644
>>>>>> --- a/arch/arm64/Kconfig
>>>>>> +++ b/arch/arm64/Kconfig
>>>>>> @@ -409,7 +409,6 @@ source "fs/Kconfig.binfmt"
>>>>>>
>>>>>>  config COMPAT
>>>>>>         bool "Kernel support for 32-bit EL0"
>>>>>> -       depends on !ARM64_64K_PAGES
>>>>>>         select COMPAT_BINFMT_ELF
>>>>>>         select HAVE_UID16
>>>>>>         select OLD_SIGSUSPEND3
>>>>>
>>>>> This is hardly "compat". Sure, it's great to have a new binutils that
>>>>> has larger alignment, but practically not a single existing binary
>>>>> will work today if someone tries to do this.
>>>>
>>>> Yes, but IMHO that's an implementation detail. The same applies for
>>>> 32bit PPC binaries if you use 4k aligned segments. If your applications
>>>> are not aligned for your page size, you can't run them. The only
>>>> platform that managed nevertheless FWIW was IA64 ;).
>>>
>>> Yes, but there the binutils change happened early enough that by the
>>> time the kernel change went in, all major distros had binaries that
>>> were compatible.
>>
>> What is the exact symptom you see when running an unaligned user
>> space binary on 64k-pages? Do we at least print a helpful error
>> message somewhere or does it just crash?
> 
> The application doesn't even start because it cannot map page 0. It
> looks like most 32-bit arm binaries are linked to be loaded at 32K. 

I looked at this briefly and the first problem I narrowed in on was underflow
in the calculation of the off variable in elf_map().

http://lxr.free-electrons.com/source/fs/binfmt_elf.c?v=3.18#L341

Chris

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