Driver Core: devtmpfs - kernel-maintained tmpfs-based /dev

Devtmpfs lets the kernel create a tmpfs instance called devtmpfs
very early at kernel initialization, before any driver-core device
is registered. Every device with a major/minor will provide a
device node in devtmpfs.

Devtmpfs can be changed and altered by userspace at any time,
and in any way needed - just like today's udev-mounted tmpfs.
Unmodified udev versions will run just fine on top of it, and will
recognize an already existing kernel-created device node and use it.
The default node permissions are root:root 0600. Proper permissions
and user/group ownership, meaningful symlinks, all other policy still
needs to be applied by userspace.

If a node is created by devtmps, devtmpfs will remove the device node
when the device goes away. If the device node was created by
userspace, or the devtmpfs created node was replaced by userspace, it
will no longer be removed by devtmpfs.

If it is requested to auto-mount it, it makes init=/bin/sh work
without any further userspace support. /dev will be fully populated
and dynamic, and always reflect the current device state of the kernel.
With the commonly used dynamic device numbers, it solves the problem
where static devices nodes may point to the wrong devices.

It is intended to make the initial bootup logic simpler and more robust,
by de-coupling the creation of the inital environment, to reliably run
userspace processes, from a complex userspace bootstrap logic to provide
a working /dev.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Jan Blunck <jblunck@suse.de>
Tested-By: Harald Hoyer <harald@redhat.com>
Tested-By: Scott James Remnant <scott@ubuntu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 3 insertions, 6 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 5a0b3d4055f3..bd20f8bb02aa 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2298,8 +2298,7 @@ static void shmem_put_super(struct super_block *sb)
         sb->s_fs_info = NULL;
 }
 
-static int shmem_fill_super(struct super_block *sb,
-                            void *data, int silent)
+int shmem_fill_super(struct super_block *sb, void *data, int silent)
 {
         struct inode *inode;
         struct dentry *root;
@@ -2519,7 +2518,7 @@ static struct file_system_type tmpfs_fs_type = {
         .kill_sb        = kill_litter_super,
 };
 
-static int __init init_tmpfs(void)
+int __init init_tmpfs(void)
 {
         int error;
 
@@ -2576,7 +2575,7 @@ static struct file_system_type tmpfs_fs_type = {
         .kill_sb        = kill_litter_super,
 };
 
-static int __init init_tmpfs(void)
+int __init init_tmpfs(void)
 {
         BUG_ON(register_filesystem(&tmpfs_fs_type) != 0);
 
@@ -2687,5 +2686,3 @@ int shmem_zero_setup(struct vm_area_struct *vma)
         vma->vm_ops = &shmem_vm_ops;
         return 0;
 }
-
-module_init(init_tmpfs)