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

[Kirill A. Shutemov]

On Fri, Dec 30, 2016 at 06:08:27PM -0800, Andy Lutomirski wrote:
> On Wed, Dec 28, 2016 at 6:53 PM, Carlos O'Donell <carlos@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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. Itâs 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.
> >>>>
> >>>>   - Itâs 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.

+ H.J.

There's discussion of proposal of "Program Properties"[1]. It seems fits
the purpose.

[1] https://sourceware.org/ml/gnu-gabi/2016-q4/msg00000.html

-- 
 Kirill A. Shutemov