[PATCH v3] powerpc: treat pkey-0 special

Ram Pai linuxram at us.ibm.com
Thu Mar 15 08:01:35 AEDT 2018

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 powerpc.

    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 <dave.hansen at intel.com>
cc: Michael Ellermen <mpe at ellerman.id.au>
cc: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Ram Pai <linuxram at us.ibm.com>
 arch/powerpc/include/asm/pkeys.h | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index 0409c80..3c1deec 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h
@@ -101,10 +101,18 @@ static inline u16 pte_to_pkey_bits(u64 pteflags)
 static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
-	/* A reserved key is never considered as 'explicitly allocated' */
-	return ((pkey < arch_max_pkey()) &&
-		!__mm_pkey_is_reserved(pkey) &&
-		__mm_pkey_is_allocated(mm, pkey));
+	/* pkey 0 is allocated by default. */
+	if (!pkey)
+	       return true;
+	if (pkey < 0 || pkey >= arch_max_pkey())
+	       return false;
+	/* Reserved keys are never allocated. */
+	if (__mm_pkey_is_reserved(pkey))
+	       return false;
+	return __mm_pkey_is_allocated(mm, pkey);
 extern void __arch_activate_pkey(int pkey);
@@ -150,7 +158,8 @@ static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
 	if (static_branch_likely(&pkey_disabled))
 		return -1;
-	if (!mm_pkey_is_allocated(mm, pkey))
+	/* pkey 0 cannot be freed */
+	if (!pkey || !mm_pkey_is_allocated(mm, pkey))
 		return -EINVAL;

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