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.