userfaultfd: convert to use anon_inode_getfd()

Nothing actually calls userfaultfd_file_create() besides the
userfaultfd() system call itself.  So simplify things by folding it into
the system call and using anon_inode_getfd() instead of
anon_inode_getfile().  Do the same in resolve_userfault_fork() as well.

This removes over 50 lines with no change in functionality.

Link: http://lkml.kernel.org/r/20171229212403.22800-1-ebiggers3@gmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -988,24 +988,14 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
 				  struct uffd_msg *msg)
 {
 	int fd;
-	struct file *file;
-	unsigned int flags = new->flags & UFFD_SHARED_FCNTL_FLAGS;
 
-	fd = get_unused_fd_flags(flags);
+	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
+			      O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
 	if (fd < 0)
 		return fd;
 
-	file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, new,
-				  O_RDWR | flags);
-	if (IS_ERR(file)) {
-		put_unused_fd(fd);
-		return PTR_ERR(file);
-	}
-
-	fd_install(fd, file);
 	msg->arg.reserved.reserved1 = 0;
 	msg->arg.fork.ufd = fd;
-
 	return 0;
 }
 
@@ -1887,24 +1877,10 @@ static void init_once_userfaultfd_ctx(void *mem)
 	seqcount_init(&ctx->refile_seq);
 }
 
-/**
- * userfaultfd_file_create - Creates a userfaultfd file pointer.
- * @flags: Flags for the userfaultfd file.
- *
- * This function creates a userfaultfd file pointer, w/out installing
- * it into the fd table. This is useful when the userfaultfd file is
- * used during the initialization of data structures that require
- * extra setup after the userfaultfd creation. So the userfaultfd
- * creation is split into the file pointer creation phase, and the
- * file descriptor installation phase.  In this way races with
- * userspace closing the newly installed file descriptor can be
- * avoided.  Returns a userfaultfd file pointer, or a proper error
- * pointer.
- */
-static struct file *userfaultfd_file_create(int flags)
+SYSCALL_DEFINE1(userfaultfd, int, flags)
 {
-	struct file *file;
 	struct userfaultfd_ctx *ctx;
+	int fd;
 
 	BUG_ON(!current->mm);
 
@@ -1912,14 +1888,12 @@ static struct file *userfaultfd_file_create(int flags)
 	BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
 	BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
 
-	file = ERR_PTR(-EINVAL);
 	if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
-		goto out;
+		return -EINVAL;
 
-	file = ERR_PTR(-ENOMEM);
 	ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
 	if (!ctx)
-		goto out;
+		return -ENOMEM;
 
 	atomic_set(&ctx->refcount, 1);
 	ctx->flags = flags;
@@ -1930,39 +1904,13 @@ static struct file *userfaultfd_file_create(int flags)
 	/* prevent the mm struct to be freed */
 	mmgrab(ctx->mm);
 
-	file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
-				  O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
-	if (IS_ERR(file)) {
+	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
+			      O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
+	if (fd < 0) {
 		mmdrop(ctx->mm);
 		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
 	}
-out:
-	return file;
-}
-
-SYSCALL_DEFINE1(userfaultfd, int, flags)
-{
-	int fd, error;
-	struct file *file;
-
-	error = get_unused_fd_flags(flags & UFFD_SHARED_FCNTL_FLAGS);
-	if (error < 0)
-		return error;
-	fd = error;
-
-	file = userfaultfd_file_create(flags);
-	if (IS_ERR(file)) {
-		error = PTR_ERR(file);
-		goto err_put_unused_fd;
-	}
-	fd_install(fd, file);
-
 	return fd;
-
-err_put_unused_fd:
-	put_unused_fd(fd);
-
-	return error;
 }
 
 static int __init userfaultfd_init(void)