Re: [PATCHv6 4/5] x86/mm: check in_compat_syscall() instead TIF_ADDR32 for mmap(MAP_32BIT)

Dmitry Safonov
2017-03-13T10:00:07Z

On 03/13/2017 12:39 PM, Thomas Gleixner wrote:
> On Mon, 6 Mar 2017, Dmitry Safonov wrote:
>
>> Result of mmap() calls with MAP_32BIT flag at this moment depends
>> on thread flag TIF_ADDR32, which is set during exec() for 32-bit apps.
>> It's broken as the behavior of mmap() shouldn't depend on exec-ed
>> application's bitness. Instead, it should check the bitness of mmap()
>> syscall.
>> How it worked before:
>> o for 32-bit compatible binaries it is completely ignored. Which was
>> fine when there were one mmap_base, computed for 32-bit syscalls.
>> After introducing mmap_compat_base 64-bit syscalls do use computed
>> for 64-bit syscalls mmap_base, which means that we can allocate 64-bit
>> address with 64-bit syscall in application launched from 32-bit
>> compatible binary. And ignoring this flag is not expected behavior.
>
> Well, the real question here is, whether we should allow 32bit applications
> to obtain 64bit mappings at all. We can very well force 32bit applications
> into the 4GB address space as it was before your mmap base splitup and be
> done with it.

Hmm, yes, we could restrict 32bit applications to 32bit mappings only.
But the approach which I tried to follow in the patches set, it was do
not base the logic on the bitness of launched applications
(native/compat) - only base on bitness of the performing syscall.
The idea was suggested by Andy and I made mmap() logic here independent
from original application's bitness.

It also seems to me simpler:
if 32-bit application wants to allocate 64-bit mapping, it should
long-jump with 64-bit segment descriptor and do `syscall` instruction
for 64-bit syscall entry path. So, in my point of view after this dance
the application does not differ much from native 64-bit binary and can
have 64-bit address mapping.

>
> Thanks,
>
> tglx
>
>

--
Dmitry