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

[Dmitry Safonov]

On 01/17/2017 11:30 PM, Andy Lutomirski wrote:
> On Mon, Jan 16, 2017 at 4:33 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> 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