Re: [PATCHv2 4/5] x86/mm: for MAP_32BIT check in_compat_syscall() instead TIF_ADDR32

[Dmitry Safonov <dsafonov@virtuozzo.com>]

On 01/17/2017 11:30 PM, Andy Lutomirski wrote:
> On Mon, Jan 16, 2017 at 4:33 AM, Dmitry Safonov <dsafonov@virtuozzo.com> wrote:
>> At this momet, logic in arch_get_unmapped_area{,_topdown} for mmaps with
>> MAP_32BIT flag checks TIF_ADDR32 which means:
>> o if 32-bit ELF changes mode to 64-bit on x86_64 and then tries to
>>   mmap() with MAP_32BIT it'll result in addr over 4Gb (as default is
>>   top-down allocation)
>> o if 64-bit ELF changes mode to 32-bit and tries mmap() with MAP_32BIT,
>>   it'll allocate only memory in 1GB space: [0x40000000, 0x80000000).
>>
>> Fix it by handeling MAP_32BIT in 64-bit syscalls only.
>> As a little bonus it'll make thread flag a little less used.
>
> Seems like an improvement.  Also, jeez, the mmap code is complicated.

Yes, but it's intentionally 4 of 5 by the reason, that it'll broke
the second case:

 >> o if 64-bit ELF changes mode to 32-bit and tries mmap() with MAP_32BIT,
 >>   it'll allocate only memory in 1GB space: [0x40000000, 0x80000000).

if the first 3 patches has not applied earlier.
(because it'll allocate not in 1Gb space, but in 64-bit whole space,
above 4Gb).
So, this should go only after fixing compat mmap in 64-bit binaries,
which is done in prev 3 patches.

-- 
              Dmitry

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