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
[off-list ref] wrote:

quoted

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.

That way setuid works right.

Um.. I probably miss background here.

-- 
 Kirill A. Shutemov

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