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

[Kirill A. Shutemov]

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.

> That way setuid works right.

Um.. I probably miss background here.

-- 
 Kirill A. Shutemov