linux-kernel - Re: [PATCHv2 4/5] x86/mm: for MAP_32BIT check in_compat_syscall() instead TIF

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Date:   Tue, 17 Jan 2017 12:30:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dmitry Safonov <dsafonov@...tuozzo.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>, X86 ML <x86@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCHv2 4/5] x86/mm: for MAP_32BIT check in_compat_syscall()
 instead TIF_ADDR32

On Mon, Jan 16, 2017 at 4:33 AM, Dmitry Safonov <dsafonov@...tuozzo.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.