[RFC][PATCH] init: Open /dev/console from rootfs

[RFC][PATCH] init: Open /dev/console from rootfs

[Eric W. Biederman]

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>
---
 init/do_mounts_initrd.c |    4 ----
 init/main.c             |   11 ++++++-----
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/init/do_mounts_initrd.c b/init/do_mounts_initrd.c
index 614241b..2b10853 100644
--- a/init/do_mounts_initrd.c
+++ b/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);
 }
 
diff --git a/init/main.c b/init/main.c
index 67e40fc..9ba505f 100644
--- a/init/main.c
+++ b/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;
 
@@ -864,6 +859,12 @@ 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
-- 
1.6.5.2.143.g8cc62

Re: [RFC][PATCH] init: Open /dev/console from rootfs

[H. Peter Anvin]

On 03/02/2010 11:53 PM, Eric W. Biederman wrote:
> 
> 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?
> 

There probably are ... but if so, they can just re-open their new
console of choice after creating it, and/or use an initramfs which
overrides the default /dev/console.

As such, this makes a lot of sense to me.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

Re: [RFC][PATCH] init: Open /dev/console from rootfs

[Al Viro]

On Tue, Mar 02, 2010 at 11:53:19PM -0800, Eric W. Biederman wrote:
> 
> 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.

Hell, yeah.  Applied, with thanks.

Re: [RFC][PATCH] init: Open /dev/console from rootfs

[H. Peter Anvin]

On 03/03/2010 11:57 AM, Al Viro wrote:
> On Tue, Mar 02, 2010 at 11:53:19PM -0800, Eric W. Biederman wrote:
>>
>> 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.
> 
> Hell, yeah.  Applied, with thanks.

:) :)

	-hpa