[PATCH] sanitize locate_fd()

* 'file' argument is unused; lose it.
* move setting flags from the caller (dupfd()) to locate_fd();
  pass cloexec flag as new argument.  Note that files_fdtable()
  that used to be in dupfd() isn't needed in the place in
  locate_fd() where the moved code ends up - we know that ->file_lock
  hadn't been dropped since the last time we calculated fdt because
  we can get there only if expand_files() returns 0 and it doesn't
  drop/reacquire in that case.
* move getting/dropping ->file_lock into locate_fd().  Now the caller
  doesn't need to do anything with files_struct *files anymore and
  we can move that inside locate_fd() as well, killing the
  struct files_struct * argument.

At that point locate_fd() is extremely similar to get_unused_fd_flags()
and the next patches will merge those two.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fcntl.c

@@ -55,14 +55,16 @@ static int get_close_on_exec(unsigned int fd)
  * file_lock held for write.
  */
 
-static int locate_fd(struct files_struct *files, 
-			    struct file *file, unsigned int orig_start)
+static int locate_fd(unsigned int orig_start, int cloexec)
 {
+	struct files_struct *files = current->files;
 	unsigned int newfd;
 	unsigned int start;
 	int error;
 	struct fdtable *fdt;
 
+	spin_lock(&files->file_lock);
+
 	error = -EINVAL;
 	if (orig_start >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)
 		goto out;
@@ -97,42 +99,28 @@ repeat:
 	if (error)
 		goto repeat;
 
-	/*
-	 * We reacquired files_lock, so we are safe as long as
-	 * we reacquire the fdtable pointer and use it while holding
-	 * the lock, no one can free it during that time.
-	 */
 	if (start <= files->next_fd)
 		files->next_fd = newfd + 1;
 
+	FD_SET(newfd, fdt->open_fds);
+	if (cloexec)
+		FD_SET(newfd, fdt->close_on_exec);
+	else
+		FD_CLR(newfd, fdt->close_on_exec);
 	error = newfd;
 
 out:
+	spin_unlock(&files->file_lock);
 	return error;
 }
 
 static int dupfd(struct file *file, unsigned int start, int cloexec)
 {
-	struct files_struct * files = current->files;
-	struct fdtable *fdt;
-	int fd;
-
-	spin_lock(&files->file_lock);
-	fd = locate_fd(files, file, start);
-	if (fd >= 0) {
-		/* locate_fd() may have expanded fdtable, load the ptr */
-		fdt = files_fdtable(files);
-		FD_SET(fd, fdt->open_fds);
-		if (cloexec)
-			FD_SET(fd, fdt->close_on_exec);
-		else
-			FD_CLR(fd, fdt->close_on_exec);
-		spin_unlock(&files->file_lock);
+	int fd = locate_fd(start, cloexec);
+	if (fd >= 0)
 		fd_install(fd, file);
-	} else {
-		spin_unlock(&files->file_lock);
+	else
 		fput(file);
-	}
 
 	return fd;
 }