init: add an init_dup helper

Add a simple helper to grab a reference to a file and install it at
the next available fd, and switch the early init code over to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/init.c

diff --git a/fs/init.c b/fs/init.c
index db5c48a..e9c320a 100644
--- a/fs/init.c
+++ b/fs/init.c
@@ -8,6 +8,7 @@
 #include <linux/namei.h>
 #include <linux/fs.h>
 #include <linux/fs_struct.h>
+#include <linux/file.h>
 #include <linux/init_syscalls.h>
 #include <linux/security.h>
 #include "internal.h"
@@ -251,3 +252,14 @@ int __init init_utimes(char *filename, struct timespec64 *ts)
 	path_put(&path);
 	return error;
 }
+
+int __init init_dup(struct file *file)
+{
+	int fd;
+
+	fd = get_unused_fd_flags(0);
+	if (fd < 0)
+		return fd;
+	fd_install(fd, get_file(file));
+	return 0;
+}

include/linux/init_syscalls.h

diff --git a/include/linux/init_syscalls.h b/include/linux/init_syscalls.h
index 3654b52..92045d1 100644
--- a/include/linux/init_syscalls.h
+++ b/include/linux/init_syscalls.h
@@ -16,3 +16,4 @@ int __init init_unlink(const char *pathname);
 int __init init_mkdir(const char *pathname, umode_t mode);
 int __init init_rmdir(const char *pathname);
 int __init init_utimes(char *filename, struct timespec64 *ts);
+int __init init_dup(struct file *file);

init/main.c

diff --git a/init/main.c b/init/main.c
index 1c710d3..9dae9c4 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1467,10 +1467,10 @@ void __init console_on_rootfs(void)
 		pr_err("Warning: unable to open an initial console.\n");
 		return;
 	}
-	get_file_rcu_many(file, 2);
-	fd_install(get_unused_fd_flags(0), file);
-	fd_install(get_unused_fd_flags(0), file);
-	fd_install(get_unused_fd_flags(0), file);
+	init_dup(file);
+	init_dup(file);
+	init_dup(file);
+	fput(file);
 }
 
 static noinline void __init kernel_init_freeable(void)