init: Open /dev/console from rootfs

To avoid potential problems with an empty /dev open /dev/console
from rootfs instead of waiting to mount our root filesystem and
mounting it there.   This effectively guarantees that there will
be a device node, and it won't be on a filesystem that we will
ever unmount, so there are no issues with leaving /dev/console
open and pinning the filesystem.

This is actually more effective than automatically mounting
devtmpfs on /dev because it removes removes the occasionally
problematic assumption that /dev/console exists from the boot
code.

With this patch I was able to throw busybox on my /boot partition
(which has no /dev directory) and boot into userspace without
problems.

The only possible negative consequence I can think of is that
someone out there deliberately used did not use a character device
that is major 5 minor 2 for /dev/console.  Does anyone know of a
situation in which that could make sense?
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

init/do_mounts_initrd.c

@@ -30,11 +30,7 @@ static int __init do_linuxrc(void * shell)
 	extern char * envp_init[];
 
 	sys_close(old_fd);sys_close(root_fd);
-	sys_close(0);sys_close(1);sys_close(2);
 	sys_setsid();
-	(void) sys_open("/dev/console",O_RDWR,0);
-	(void) sys_dup(0);
-	(void) sys_dup(0);
 	return kernel_execve(shell, argv, envp_init);
 }
 

init/main.c

@@ -806,11 +806,6 @@ static noinline int init_post(void)
 	system_state = SYSTEM_RUNNING;
 	numa_default_policy();
 
-	if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
-		printk(KERN_WARNING "Warning: unable to open an initial console.\n");
-
-	(void) sys_dup(0);
-	(void) sys_dup(0);
 
 	current->signal->flags |= SIGNAL_UNKILLABLE;
 
@@ -873,6 +868,12 @@ static int __init kernel_init(void * unused)
 
 	do_basic_setup();
 
+	/* Open the /dev/console on the rootfs, this should never fail */
+	if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
+		printk(KERN_WARNING "Warning: unable to open an initial console.\n");
+
+	(void) sys_dup(0);
+	(void) sys_dup(0);
 	/*
 	 * check if there is an early userspace init.  If yes, let it do all
 	 * the work