On 06/11/2016 02:47 AM, Thomas Gleixner wrote:

On Wed, 8 Jun 2016, Dave Hansen wrote:

quoted

quoted

Proposed semantics:
1. protection key 0 is special and represents the default,
   unassigned protection key.  It is always allocated.
2. mprotect() never affects a mapping's pkey_mprotect()-assigned
   protection key. A protection key of 0 (even if set explicitly)
   represents an unassigned protection key.
   2a. mprotect(PROT_EXEC) on a mapping with an assigned protection
       key may or may not result in a mapping with execute-only
       properties.  pkey_mprotect() plus pkey_set() on all threads
       should be used to _guarantee_ execute-only semantics.
3. mprotect(PROT_EXEC) may result in an "execute-only" mapping. The
   kernel will internally attempt to allocate and dedicate a
   protection key for the purpose of execute-only mappings.  This
   may not be possible in cases where there are no free protection
   keys available.

Shouldn't we just reserve a protection key for PROT_EXEC unconditionally?

Normal userspace does not do PROT_EXEC today.  So, today, we'd
effectively lose one of our keys by reserving it.  Of the folks I've
talked to who really want this feature, and *will* actually use it, one
of the most common complaints is that there are too few keys.

Folks who actively *want* true PROT_EXEC semantics can use the explicit
pkey interfaces.