Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

["Carlos O'Donell" <carlos@redhat.com>]

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@linux.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?"

-- 
Cheers,
Carlos.