userfaultfd: Rename uffd_api.bits into .features

This is (seems to be) the minimal thing that is required to unblock
standard uffd usage from the non-cooperative one. Now more bits can be
added to the features field indicating e.g. UFFD_FEATURE_FORK and others
needed for the latter use-case.

Signed-off-by: Pavel Emelyanov <[e-mail]>
Signed-off-by: Andrea Arcangeli <[e-mail]>
Cc: Sanidhya Kashyap <[e-mail]>
Cc: [e-mail]
Cc: "Kirill A. Shutemov" <[e-mail]>
Cc: Andres Lagar-Cavilla <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Peter Feiner <[e-mail]>
Cc: "Dr. David Alan Gilbert" <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: "Huangpeng (Peter)" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/userfaultfd.c

···
 		goto out;
 	}
 	/* careful not to leak info, we only read the first 8 bytes */
-	uffdio_api.bits = UFFD_API_BITS;
+	uffdio_api.features = UFFD_API_FEATURES;
 	uffdio_api.ioctls = UFFD_API_IOCTLS;
 	ret = -EFAULT;
 	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
···
 	 *	protocols: aa:... bb:...
 	 */
 	seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
-		   pending, total, UFFD_API, UFFD_API_BITS,
+		   pending, total, UFFD_API, UFFD_API_FEATURES,
 		   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
 }
 #endif

include/uapi/linux/userfaultfd.h

···
 #include <linux/types.h>
 
 #define UFFD_API ((__u64)0xAA)
-/* FIXME: add "|UFFD_BIT_WP" to UFFD_API_BITS after implementing it */
-#define UFFD_API_BITS (UFFD_BIT_WRITE)
+/* FIXME: add "|UFFD_FEATURE_WP" to UFFD_API_FEATURES after implementing it */
+#define UFFD_API_FEATURES (UFFD_FEATURE_WRITE_BIT)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
···
 #define UFFD_BIT_WP	(1<<1)	/* handle_userfault() reason VM_UFFD_WP */
 #define UFFD_BITS	2	/* two above bits used for UFFD_BIT_* mask */
 
+/*
+ * Features reported in uffdio_api.features field
+ */
+#define UFFD_FEATURE_WRITE_BIT	(1<<0) /* Corresponds to UFFD_BIT_WRITE */
+#define UFFD_FEATURE_WP_BIT	(1<<1) /* Corresponds to UFFD_BIT_WP */
+
 struct uffdio_api {
 	/* userland asks for an API number */
 	__u64 api;
 
 	/* kernel answers below with the available features for the API */
-	__u64 bits;
+	__u64 features;
 	__u64 ioctls;
 };