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

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Date:   Fri, 30 Dec 2016 18:08:27 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "Carlos O'Donell" <carlos@...hat.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        "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>,
        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>
Subject: Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

On Wed, Dec 28, 2016 at 6:53 PM, Carlos O'Donell <carlos@...hat.com> wrote:
> On 12/26/2016 09:24 PM, Kirill A. Shutemov wrote:
>> On Mon, Dec 26, 2016 at 06:06:01PM -0800, Andy Lutomirski wrote:
>>> On Mon, Dec 26, 2016 at 5:54 PM, Kirill A. Shutemov
>>> <kirill.shutemov@...ux.intel.com> wrote:
>>>> This patch introduces new rlimit resource to manage maximum virtual
>>>> address available to userspace to map.
>>>>
>>>> On x86, 5-level paging enables 56-bit userspace virtual address space.
>>>> Not all user space is ready to handle wide addresses. It's known that
>>>> at least some JIT compilers use high bit in pointers to encode their
>>>> information. It collides with valid pointers with 5-level paging and
>>>> leads to crashes.
>>>>
>>>> The patch aims to address this compatibility issue.
>>>>
>>>> MM would use min(RLIMIT_VADDR, TASK_SIZE) as upper limit of virtual
>>>> address available to map by userspace.
>>>>
>>>> The default hard limit will be RLIM_INFINITY, which basically means that
>>>> TASK_SIZE limits available address space.
>>>>
>>>> The soft limit will also be RLIM_INFINITY everywhere, but the machine
>>>> with 5-level paging enabled. In this case, soft limit would be
>>>> (1UL << 47) - PAGE_SIZE. It’s current x86-64 TASK_SIZE_MAX with 4-level
>>>> paging which known to be safe
>>>>
>>>> New rlimit resource would follow usual semantics with regards to
>>>> inheritance: preserved on fork(2) and exec(2). This has potential to
>>>> break application if limits set too wide or too narrow, but this is not
>>>> uncommon for other resources (consider RLIMIT_DATA or RLIMIT_AS).
>>>>
>>>> As with other resources you can set the limit lower than current usage.
>>>> It would affect only future virtual address space allocations.
>>>>
>>>> Use-cases for new rlimit:
>>>>
>>>>   - Bumping the soft limit to RLIM_INFINITY, allows current process all
>>>>     its children to use addresses above 47-bits.
>>>>
>>>>   - Bumping the soft limit to RLIM_INFINITY after fork(2), but before
>>>>     exec(2) allows the child to use addresses above 47-bits.
>>>>
>>>>   - Lowering the hard limit to 47-bits would prevent current process all
>>>>     its children to use addresses above 47-bits, unless a process has
>>>>     CAP_SYS_RESOURCES.
>>>>
>>>>   - It’s also can be handy to lower hard or soft limit to arbitrary
>>>>     address. User-mode emulation in QEMU may lower the limit to 32-bit
>>>>     to emulate 32-bit machine on 64-bit host.
>>>
>>> I tend to think that this should be a personality or an ELF flag, not
>>> an rlimit.
>>
>> My plan was to implement ELF flag on top. Basically, ELF flag would mean
>> that we bump soft limit to hard limit on exec.
>
> Could you clarify what you mean by an "ELF flag?"

Some way to mark a binary as supporting a larger address space.  I
don't have a precise solution in mind, but an ELF note might be a good
way to go here.

--Andy