linux-kernel - [PATCH v3] x86: treat pkey-0 special

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Date:   Wed, 14 Mar 2018 14:00:14 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     mingo@...hat.com
Cc:     mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, x86@...nel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, benh@...nel.crashing.org, paulus@...ba.org,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, hbabu@...ibm.com, mhocko@...nel.org,
        bauerman@...ux.vnet.ibm.com, ebiederm@...ssion.com,
        linuxram@...ibm.com, corbet@....net, arnd@...db.de,
        fweimer@...hat.com, msuchanek@...e.com, Ulrich.Weigand@...ibm.com
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.

History:
    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@...el.com>
cc: Michael Ellermen <mpe@...erman.id.au>
cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ram Pai <linuxram@...ibm.com>
---
 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);
-- 
1.8.3.1