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

From: [email-masked] (Alexander Graf)
Date: Fri, 05 Dec 2014 13:06:10 +0100

On 05.12.14 11:39, 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?

It simply doesn't start:

init-4.2# /hello.binutils-2.23
init-4.2# echo $?
139
init-4.2# /hello.binutils-2.25

Hello world!

init-4.2# echo $?
0
init-4.2#

I'm not sure how to give the user an actually helpful error output here
though. The only real handle we have for executing a binary is to return
an error code.

Alex