linux-kernel - Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT

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Message-ID: <CALCETrXmdAnbgjsxw=qTbP2PhVCzE8v3y5XMt8DVa4P9DowsGA@mail.gmail.com>
Date:   Tue, 3 Jan 2017 14:09:16 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Arnd Bergmann <arnd@...db.de>
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>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.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>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

On Tue, Jan 3, 2017 at 2:07 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tuesday, January 3, 2017 10:29:33 AM CET Andy Lutomirski wrote:
>>
>> Hmm.  What if we approached this a bit differently?  We could add a
>> single new personality bit ADDR_LIMIT_EXPLICIT.  Setting this bit
>> cause PER_LINUX32_3GB etc to be automatically cleared.
>
> Both the ADDR_LIMIT_32BIT and ADDR_LIMIT_3GB flags I guess?

Yes.

>
>> When
>> ADDR_LIMIT_EXPLICIT is in effect, prctl can set a 64-bit numeric
>> limit.  If ADDR_LIMIT_EXPLICIT is cleared, the prctl value stops being
>> settable and reading it via prctl returns whatever is implied by the
>> other personality bits.
>
> I don't see anything wrong with it, but I'm a bit confused now
> what this would be good for, compared to using just prctl.
>
> Is this about setuid clearing the personality but not the prctl,
> or something else?

It's to avid ambiguity as to what happens if you set ADDR_LIMIT_32BIT
and use the prctl.  ISTM it would be nice for the semantics to be
fully defined in all cases.

--Andy