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Subject[PATCH v3] x86: treat pkey-0 special
Applications need the ability to associate an address-range with some
key and latter revert to its initial default key. Pkey-0 comes close to
providing this function but falls short, because the current
implementation disallows applications to explicitly associate pkey-0 to
the address range.

This patch clarifies the semantics of pkey-0 and provides the
corresponding implementation on powerpc.

Pkey-0 is special with the following semantics.
(a) it is implicitly allocated and can never be freed. It always exists.
(b) it is the default key assigned to any address-range.
(c) it can be explicitly associated with any address-range.

Tested on x86_64.

v3 : added clarification of the semantics of pkey0.
-- suggested by Dave Hansen
v2 : split the patch into two, one for x86 and one for powerpc
-- suggested by Michael Ellermen

cc: Dave Hansen <>
cc: Michael Ellermen <>
cc: Ingo Molnar <>
Signed-off-by: Ram Pai <>
arch/x86/include/asm/pkeys.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
index a0ba1ff..6ea7486 100644
--- a/arch/x86/include/asm/pkeys.h
+++ b/arch/x86/include/asm/pkeys.h
@@ -52,7 +52,7 @@ bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
* from pkey_alloc(). pkey 0 is special, and never
* returned from pkey_alloc().
- if (pkey <= 0)
+ if (pkey < 0)
return false;
if (pkey >= arch_max_pkey())
return false;
@@ -92,7 +92,8 @@ int mm_pkey_alloc(struct mm_struct *mm)
static inline
int mm_pkey_free(struct mm_struct *mm, int pkey)
- if (!mm_pkey_is_allocated(mm, pkey))
+ /* pkey 0 is special and can never be freed */
+ if (!pkey || !mm_pkey_is_allocated(mm, pkey))
return -EINVAL;

mm_set_pkey_free(mm, pkey);
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