linux-kernel - Re: [PATCHv2 3/5] x86/mm: fix native mmap() in compat bins and vice-versa

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Date:   Tue, 17 Jan 2017 12:29:28 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dmitry Safonov <dsafonov@...tuozzo.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 3/5] x86/mm: fix native mmap() in compat bins and vice-versa

On Mon, Jan 16, 2017 at 4:33 AM, Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
> Fix 32-bit compat_sys_mmap() mapping VMA over 4Gb in 64-bit binaries
> and 64-bit sys_mmap() mapping VMA only under 4Gb in 32-bit binaries.
> Changed arch_get_unmapped_area{,_topdown}() to recompute mmap_base
> for those cases and use according high/low limits for vm_unmapped_area()
> The recomputing of mmap_base may make compat sys_mmap() in 64-bit
> binaries a little slower than native, which uses already known from exec
> time mmap_base - but, as it returned buggy address, that case seemed
> unused previously, so no performance degradation for already used ABI.

This looks plausibly correct but rather weird -- why does this code
need to distinguish between all four cases (pure 32-bit, pure 64-bit,
64-bit mmap layout doing 32-bit call, 32-bit layout doing 64-bit
call)?

> Can be optimized in future by introducing mmap_compat_{,legacy}_base
> in mm_struct.

Hmm.  Would it make sense to do it this way from the beginning?

If adding an in_32bit_syscall() helper would help, then by all means
please do so.

--Andy