[RFC 3/4] arm64/mm: Allow non-exec to exec transition in ptep_set_access_flags()

[Anshuman Khandual <anshuman.khandual@arm.com>]

ptep_set_access_flags() updates page table for a mapped page entry which
still got a fault probably because of a different permission than what
it is mapped with. Previously an exec enabled page always gets required
permission in the page table entry. Hence ptep_set_access_flags() never
had to move an entry from non-exec to exec. This is going to change with
deferred exec permission setting with later patches. Hence allow non-exec
to exec transition here and do the required I-cache invalidation.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/mm/fault.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 591670d..1540fc1 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -227,22 +227,25 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 	if (pte_same(pte, entry))
 		return 0;
 
-	/* only preserve the access flags and write permission */
-	pte_val(entry) &= PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY;
+	/* only preserve the access flags, write and exec permission */
+	pte_val(entry) &= PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY | PTE_UXN;
+
+	if (pte_user_exec(entry))
+		__sync_icache_dcache(pte);
 
 	/*
 	 * Setting the flags must be done atomically to avoid racing with the
-	 * hardware update of the access/dirty state. The PTE_RDONLY bit must
-	 * be set to the most permissive (lowest value) of *ptep and entry
-	 * (calculated as: a & b == ~(~a | ~b)).
+	 * hardware update of the access/dirty state. The PTE_RDONLY bit and
+	 * PTE_UXN must be set to the most permissive (lowest value) of *ptep
+	 * and entry (calculated as: a & b == ~(~a | ~b)).
 	 */
-	pte_val(entry) ^= PTE_RDONLY;
+	pte_val(entry) ^= PTE_RDONLY | PTE_UXN;
 	pteval = pte_val(pte);
 	do {
 		old_pteval = pteval;
-		pteval ^= PTE_RDONLY;
+		pteval ^= PTE_RDONLY | PTE_UXN;
 		pteval |= pte_val(entry);
-		pteval ^= PTE_RDONLY;
+		pteval ^= PTE_RDONLY | PTE_UXN;
 		pteval = cmpxchg_relaxed(&pte_val(*ptep), old_pteval, pteval);
 	} while (pteval != old_pteval);
 
-- 
2.7.4