[RFC 4/8] userfaultfd: allow userfaultfd register success with writeprotection

[Shaohua Li <shli@fb.com>]

The userfaultfd register ioctl currently disables writeprotection.
Enable it.

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Shaohua Li <shli@fb.com>
---
 fs/userfaultfd.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index eaa5086..12176b5 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -236,6 +236,8 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 	 */
 	if (pte_none(*pte))
 		ret = true;
+	if (!pte_write(*pte) && (reason & VM_UFFD_WP))
+		ret = true;
 	pte_unmap(pte);
 
 out:
@@ -736,15 +738,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 	vm_flags = 0;
 	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
 		vm_flags |= VM_UFFD_MISSING;
-	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP)
 		vm_flags |= VM_UFFD_WP;
-		/*
-		 * FIXME: remove the below error constraint by
-		 * implementing the wprotect tracking mode.
-		 */
-		ret = -EINVAL;
-		goto out;
-	}
 
 	ret = validate_range(mm, uffdio_register.range.start,
 			     uffdio_register.range.len);
@@ -784,6 +779,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 		ret = -EINVAL;
 		if (cur->vm_ops)
 			goto out_unlock;
+		if ((vm_flags & VM_UFFD_WP) && !(cur->vm_flags & VM_WRITE))
+			goto out_unlock;
 
 		/*
 		 * Check that this vma isn't already owned by a
-- 
2.4.6

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