add struct fd constructors, get rid of __to_fd()

	Make __fdget() et.al. return struct fd directly.
New helpers: BORROWED_FD(file) and CLONED_FD(file), for
borrowed and cloned file references resp.

	NOTE: this might need tuning; in particular, inline on
__fget_light() is there to keep the code generation same as
before - we probably want to keep it inlined in fdget() et.al.
(especially so in fdget_pos()), but that needs profiling.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/file.c

@@ -1128,7 +1128,7 @@ EXPORT_SYMBOL(task_lookup_next_fdget_rcu);
  * The fput_needed flag returned by fget_light should be passed to the
  * corresponding fput_light.
  */
-static unsigned long __fget_light(unsigned int fd, fmode_t mask)
+static inline struct fd __fget_light(unsigned int fd, fmode_t mask)
 {
 	struct files_struct *files = current->files;
 	struct file *file;
@@ -1145,22 +1145,22 @@ static unsigned long __fget_light(unsigned int fd, fmode_t mask)
 	if (likely(atomic_read_acquire(&files->count) == 1)) {
 		file = files_lookup_fd_raw(files, fd);
 		if (!file || unlikely(file->f_mode & mask))
-			return 0;
-		return (unsigned long)file;
+			return EMPTY_FD;
+		return BORROWED_FD(file);
 	} else {
 		file = __fget_files(files, fd, mask);
 		if (!file)
-			return 0;
-		return FDPUT_FPUT | (unsigned long)file;
+			return EMPTY_FD;
+		return CLONED_FD(file);
 	}
 }
-unsigned long __fdget(unsigned int fd)
+struct fd fdget(unsigned int fd)
 {
 	return __fget_light(fd, FMODE_PATH);
 }
-EXPORT_SYMBOL(__fdget);
+EXPORT_SYMBOL(fdget);
 
-unsigned long __fdget_raw(unsigned int fd)
+struct fd fdget_raw(unsigned int fd)
 {
 	return __fget_light(fd, 0);
 }
@@ -1181,16 +1181,16 @@ static inline bool file_needs_f_pos_lock(struct file *file)
 		(file_count(file) > 1 || file->f_op->iterate_shared);
 }
 
-unsigned long __fdget_pos(unsigned int fd)
+struct fd fdget_pos(unsigned int fd)
 {
-	unsigned long v = __fdget(fd);
-	struct file *file = (struct file *)(v & ~3);
+	struct fd f = fdget(fd);
+	struct file *file = fd_file(f);
 
 	if (file && file_needs_f_pos_lock(file)) {
-		v |= FDPUT_POS_UNLOCK;
+		f.word |= FDPUT_POS_UNLOCK;
 		mutex_lock(&file->f_pos_lock);
 	}
-	return v;
+	return f;
 }
 
 void __f_unlock_pos(struct file *f)

include/linux/file.h

@@ -53,6 +53,14 @@ static inline bool fd_empty(struct fd f)
 }
 
 #define EMPTY_FD (struct fd){0}
+static inline struct fd BORROWED_FD(struct file *f)
+{
+	return (struct fd){(unsigned long)f};
+}
+static inline struct fd CLONED_FD(struct file *f)
+{
+	return (struct fd){(unsigned long)f | FDPUT_FPUT};
+}
 
 static inline void fdput(struct fd fd)
 {
@@ -63,30 +71,11 @@ static inline void fdput(struct fd fd)
 extern struct file *fget(unsigned int fd);
 extern struct file *fget_raw(unsigned int fd);
 extern struct file *fget_task(struct task_struct *task, unsigned int fd);
-extern unsigned long __fdget(unsigned int fd);
-extern unsigned long __fdget_raw(unsigned int fd);
-extern unsigned long __fdget_pos(unsigned int fd);
 extern void __f_unlock_pos(struct file *);
 
-static inline struct fd __to_fd(unsigned long v)
-{
-	return (struct fd){v};
-}
-
-static inline struct fd fdget(unsigned int fd)
-{
-	return __to_fd(__fdget(fd));
-}
-
-static inline struct fd fdget_raw(unsigned int fd)
-{
-	return __to_fd(__fdget_raw(fd));
-}
-
-static inline struct fd fdget_pos(int fd)
-{
-	return __to_fd(__fdget_pos(fd));
-}
+struct fd fdget(unsigned int fd);
+struct fd fdget_raw(unsigned int fd);
+struct fd fdget_pos(unsigned int fd);
 
 static inline void fdput_pos(struct fd f)
 {