linux-kernel - Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX

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Message-ID: <CALCETrU957t7RQOAUJKWdjcA8t2ScWGkrnou2+cHksetC9aN=A@mail.gmail.com>
Date:   Fri, 17 Feb 2017 09:21:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX_VADDR

On Fri, Feb 17, 2017 at 9:19 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 02/17/2017 06:13 AM, Kirill A. Shutemov wrote:
>> +/*
>> + * Default maximum virtual address. This is required for
>> + * compatibility with applications that assumes 47-bit VA.
>> + * The limit can be changed with prctl(PR_SET_MAX_VADDR).
>> + */
>> +#define MAX_VADDR_DEFAULT    ((1UL << 47) - PAGE_SIZE)
>
> This is a bit goofy.  It's not the largest virtual adddress that can be
> accessed, but the beginning of the last page.

No, it really is the limit.  We don't allow user code to map the last
page because ti would be a root hole due to SYSRET.  Thanks, Intel.
See the comment near TASK_SIZE_MAX IIRC.

--Andy