Re: linux-next: x86: Unalbe to run x32 processes on the x86_64 kernel

[Andy Lutomirski]

On Tue, Mar 21, 2017 at 8:59 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote:
> On 03/21/2017 03:50 PM, Dmitry Safonov wrote:
>>
>> On 03/21/2017 03:49 PM, Thomas Gleixner wrote:
>>>
>>> On Tue, 21 Mar 2017, Adam Borowski wrote:
>>>>
>>>> On Tue, Mar 21, 2017 at 07:45:39AM +0100, Ingo Molnar wrote:
>>>>>
>>>>> * Andrei Vagin <avagin@xxxxxxxxx> wrote:
>>>>>
>>>>>> # first bad commit: [45fc8757d1d2128e342b4e7ef39adedf7752faac] x86:
>>>>>> Make the GDT remapping read-only on 64-bit
>>>>>
>>>>>
>>>>> Just wondering, does the following commit fix it:
>>>>>
>>>>>   5b781c7e317f x86/tls: Forcibly set the accessed bit in TLS segments
>>>>
>>>>
>>>> It does fix i386 but not x32.
>>>>
>>>> By "x32" I mean CONFIG_X86_X32, by "i386" CONFIG_IA32_EMULATION,
>>>> contrary to
>>>> Andrei's first report.  The naming of the new ABI wasn't too
>>>> fortunate...
>>>
>>>
>>> The X32 issue is unrelated to the GDT mapping.
>>>
>>> What happens is that the mmap rework from Dmitry switched X32 to use
>>> 64bit
>>> mappings, which is wrong. X32 has 64bit instructions and syscalls and
>>> 32bit
>>> address space.
>>
>>
>> Hmm, in_compat_syscall() checks x32 syscall bit.
>
>
> Which is not set during exec() for x32. So in_compat_syscall() doesn't
> work there.
> I've tested this patch on x32-debian port:
> https://lkml.org/lkml/2017/3/21/489

Seems generally reasonable to me.  It aligns x32 with existing
practice for i386, I think.

>
> Though I'm not very happy with the resulting patch :(
> Maybe one could suggest a better idea..

IMO it would be nice if execve() didn't call into any function that
checked in_compat_syscall(), etc, but maybe that's a pipe dream.