userfaultfd: Rename uffd_api.bits into .features

This is (seem 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 <xemul@parallels.com>

fs/userfaultfd.c

@@ -856,7 +856,7 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 		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)))
@@ -913,7 +913,7 @@ static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
 	 *	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

@@ -11,7 +11,7 @@
 
 #define UFFD_API ((__u64)0xAA)
 /* FIXME: add "|UFFD_BIT_WP" to UFFD_API_BITS after implementing it */
-#define UFFD_API_BITS (UFFD_BIT_WRITE)
+#define UFFD_API_FEATURES (UFFD_FEATURE_WRITE_BIT)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -51,12 +51,18 @@
 #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;
 };