1 files changed, 44 insertions, 0 deletions
diff --git a/fs/file.c b/fs/file.c
index 4ce4a0fcf320..78cf88f2a0e8 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -569,3 +569,47 @@ int get_unused_fd_flags(unsigned flags)
 	return __alloc_fd(current->files, 0, rlimit(RLIMIT_NOFILE), flags);
 }
 EXPORT_SYMBOL(get_unused_fd_flags);
+
+static void __put_unused_fd(struct files_struct *files, unsigned int fd)
+{
+	struct fdtable *fdt = files_fdtable(files);
+	__clear_open_fd(fd, fdt);
+	if (fd < files->next_fd)
+		files->next_fd = fd;
+}
+
+void put_unused_fd(unsigned int fd)
+{
+	struct files_struct *files = current->files;
+	spin_lock(&files->file_lock);
+	__put_unused_fd(files, fd);
+	spin_unlock(&files->file_lock);
+}
+
+EXPORT_SYMBOL(put_unused_fd);
+
+/*
+ * Install a file pointer in the fd array.
+ *
+ * The VFS is full of places where we drop the files lock between
+ * setting the open_fds bitmap and installing the file in the file
+ * array.  At any such point, we are vulnerable to a dup2() race
+ * installing a file in the array before us.  We need to detect this and
+ * fput() the struct file we are about to overwrite in this case.
+ *
+ * It should never happen - if we allow dup2() do it, _really_ bad things
+ * will follow.
+ */
+
+void fd_install(unsigned int fd, struct file *file)
+{
+	struct files_struct *files = current->files;
+	struct fdtable *fdt;
+	spin_lock(&files->file_lock);
+	fdt = files_fdtable(files);
+	BUG_ON(fdt->fd[fd] != NULL);
+	rcu_assign_pointer(fdt->fd[fd], file);
+	spin_unlock(&files->file_lock);
+}
+
+EXPORT_SYMBOL(fd_install);