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

[Andy Lutomirski]

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.