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

Dmitry Safonov
2017-03-21T16:38:01Z

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@gmail.com> 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

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

> I'll take a look, what happens there.
>
>> Dmitry, can you please have a look and fix that up? For reproduction just
>> compile helloworld.c with gcc -mx32.
>
> Sure, thanks.
>

--
Dmitry