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, 367 insertions, 0 deletions

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
new file mode 100644
index 000000000000..fd488ad4263a
--- /dev/null
+++ b/drivers/base/devtmpfs.c
@@ -0,0 +1,367 @@
+/*
+ * devtmpfs - kernel-maintained tmpfs-based /dev
+ *
+ * Copyright (C) 2009, Kay Sievers <kay.sievers@vrfy.org>
+ *
+ * During bootup, before any driver core device is registered,
+ * devtmpfs, a tmpfs-based filesystem is created. Every driver-core
+ * device which requests a device node, will add a node in this
+ * filesystem. The node is named after the the name of the device,
+ * or the susbsytem can provide a custom name. All devices are
+ * owned by root and have a mode of 0600.
+ */
+
+#include <linux/kernel.h>
+#include <linux/syscalls.h>
+#include <linux/mount.h>
+#include <linux/device.h>
+#include <linux/genhd.h>
+#include <linux/namei.h>
+#include <linux/fs.h>
+#include <linux/shmem_fs.h>
+#include <linux/cred.h>
+#include <linux/init_task.h>
+
+static struct vfsmount *dev_mnt;
+
+#if defined CONFIG_DEVTMPFS_MOUNT
+static int dev_mount = 1;
+#else
+static int dev_mount;
+#endif
+
+static int __init mount_param(char *str)
+{
+        dev_mount = simple_strtoul(str, NULL, 0);
+        return 1;
+}
+__setup("devtmpfs.mount=", mount_param);
+
+static int dev_get_sb(struct file_system_type *fs_type, int flags,
+                      const char *dev_name, void *data, struct vfsmount *mnt)
+{
+        return get_sb_single(fs_type, flags, data, shmem_fill_super, mnt);
+}
+
+static struct file_system_type dev_fs_type = {
+        .name = "devtmpfs",
+        .get_sb = dev_get_sb,
+        .kill_sb = kill_litter_super,
+};
+
+#ifdef CONFIG_BLOCK
+static inline int is_blockdev(struct device *dev)
+{
+        return dev->class == &block_class;
+}
+#else
+static inline int is_blockdev(struct device *dev) { return 0; }
+#endif
+
+static int dev_mkdir(const char *name, mode_t mode)
+{
+        struct nameidata nd;
+        struct dentry *dentry;
+        int err;
+
+        err = vfs_path_lookup(dev_mnt->mnt_root, dev_mnt,
+                              name, LOOKUP_PARENT, &nd);
+        if (err)
+                return err;
+
+        dentry = lookup_create(&nd, 1);
+        if (!IS_ERR(dentry)) {
+                err = vfs_mkdir(nd.path.dentry->d_inode, dentry, mode);
+                dput(dentry);
+        } else {
+                err = PTR_ERR(dentry);
+        }
+        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
+
+        path_put(&nd.path);
+        return err;
+}
+
+static int create_path(const char *nodepath)
+{
+        char *path;
+        struct nameidata nd;
+        int err = 0;
+
+        path = kstrdup(nodepath, GFP_KERNEL);
+        if (!path)
+                return -ENOMEM;
+
+        err = vfs_path_lookup(dev_mnt->mnt_root, dev_mnt,
+                              path, LOOKUP_PARENT, &nd);
+        if (err == 0) {
+                struct dentry *dentry;
+
+                /* create directory right away */
+                dentry = lookup_create(&nd, 1);
+                if (!IS_ERR(dentry)) {
+                        err = vfs_mkdir(nd.path.dentry->d_inode,
+                                        dentry, 0755);
+                        dput(dentry);
+                }
+                mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
+
+                path_put(&nd.path);
+        } else if (err == -ENOENT) {
+                char *s;
+
+                /* parent directories do not exist, create them */
+                s = path;
+                while (1) {
+                        s = strchr(s, '/');
+                        if (!s)
+                                break;
+                        s[0] = '\0';
+                        err = dev_mkdir(path, 0755);
+                        if (err && err != -EEXIST)
+                                break;
+                        s[0] = '/';
+                        s++;
+                }
+        }
+
+        kfree(path);
+        return err;
+}
+
+int devtmpfs_create_node(struct device *dev)
+{
+        const char *tmp = NULL;
+        const char *nodename;
+        const struct cred *curr_cred;
+        mode_t mode;
+        struct nameidata nd;
+        struct dentry *dentry;
+        int err;
+
+        if (!dev_mnt)
+                return 0;
+
+        nodename = device_get_nodename(dev, &tmp);
+        if (!nodename)
+                return -ENOMEM;
+
+        if (is_blockdev(dev))
+                mode = S_IFBLK|0600;
+        else
+                mode = S_IFCHR|0600;
+
+        curr_cred = override_creds(&init_cred);
+        err = vfs_path_lookup(dev_mnt->mnt_root, dev_mnt,
+                              nodename, LOOKUP_PARENT, &nd);
+        if (err == -ENOENT) {
+                /* create missing parent directories */
+                create_path(nodename);
+                err = vfs_path_lookup(dev_mnt->mnt_root, dev_mnt,
+                                      nodename, LOOKUP_PARENT, &nd);
+                if (err)
+                        goto out;
+        }
+
+        dentry = lookup_create(&nd, 0);
+        if (!IS_ERR(dentry)) {
+                err = vfs_mknod(nd.path.dentry->d_inode,
+                                dentry, mode, dev->devt);
+                /* mark as kernel created inode */
+                if (!err)
+                        dentry->d_inode->i_private = &dev_mnt;
+                dput(dentry);
+        } else {
+                err = PTR_ERR(dentry);
+        }
+        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
+
+        path_put(&nd.path);
+out:
+        kfree(tmp);
+        revert_creds(curr_cred);
+        return err;
+}
+
+static int dev_rmdir(const char *name)
+{
+        struct nameidata nd;
+        struct dentry *dentry;
+        int err;
+
+        err = vfs_path_lookup(dev_mnt->mnt_root, dev_mnt,
+                              name, LOOKUP_PARENT, &nd);
+        if (err)
+                return err;
+
+        mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
+        dentry = lookup_one_len(nd.last.name, nd.path.dentry, nd.last.len);
+        if (!IS_ERR(dentry)) {
+                if (dentry->d_inode)
+                        err = vfs_rmdir(nd.path.dentry->d_inode, dentry);
+                else
+                        err = -ENOENT;
+                dput(dentry);
+        } else {
+                err = PTR_ERR(dentry);
+        }
+        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
+
+        path_put(&nd.path);
+        return err;
+}
+
+static int delete_path(const char *nodepath)
+{
+        const char *path;
+        int err = 0;
+
+        path = kstrdup(nodepath, GFP_KERNEL);
+        if (!path)
+                return -ENOMEM;
+
+        while (1) {
+                char *base;
+
+                base = strrchr(path, '/');
+                if (!base)
+                        break;
+                base[0] = '\0';
+                err = dev_rmdir(path);
+                if (err)
+                        break;
+        }
+
+        kfree(path);
+        return err;
+}
+
+static int dev_mynode(struct device *dev, struct inode *inode, struct kstat *stat)
+{
+        /* did we create it */
+        if (inode->i_private != &dev_mnt)
+                return 0;
+
+        /* does the dev_t match */
+        if (is_blockdev(dev)) {
+                if (!S_ISBLK(stat->mode))
+                        return 0;
+        } else {
+                if (!S_ISCHR(stat->mode))
+                        return 0;
+        }
+        if (stat->rdev != dev->devt)
+                return 0;
+
+        /* ours */
+        return 1;
+}
+
+int devtmpfs_delete_node(struct device *dev)
+{
+        const char *tmp = NULL;
+        const char *nodename;
+        const struct cred *curr_cred;
+        struct nameidata nd;
+        struct dentry *dentry;
+        struct kstat stat;
+        int deleted = 1;
+        int err;
+
+        if (!dev_mnt)
+                return 0;
+
+        nodename = device_get_nodename(dev, &tmp);
+        if (!nodename)
+                return -ENOMEM;
+
+        curr_cred = override_creds(&init_cred);
+        err = vfs_path_lookup(dev_mnt->mnt_root, dev_mnt,
+                              nodename, LOOKUP_PARENT, &nd);
+        if (err)
+                goto out;
+
+        mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
+        dentry = lookup_one_len(nd.last.name, nd.path.dentry, nd.last.len);
+        if (!IS_ERR(dentry)) {
+                if (dentry->d_inode) {
+                        err = vfs_getattr(nd.path.mnt, dentry, &stat);
+                        if (!err && dev_mynode(dev, dentry->d_inode, &stat)) {
+                                err = vfs_unlink(nd.path.dentry->d_inode,
+                                                 dentry);
+                                if (!err || err == -ENOENT)
+                                        deleted = 1;
+                        }
+                } else {
+                        err = -ENOENT;
+                }
+                dput(dentry);
+        } else {
+                err = PTR_ERR(dentry);
+        }
+        mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
+
+        path_put(&nd.path);
+        if (deleted && strchr(nodename, '/'))
+                delete_path(nodename);
+out:
+        kfree(tmp);
+        revert_creds(curr_cred);
+        return err;
+}
+
+/*
+ * If configured, or requested by the commandline, devtmpfs will be
+ * auto-mounted after the kernel mounted the root filesystem.
+ */
+int devtmpfs_mount(const char *mountpoint)
+{
+        struct path path;
+        int err;
+
+        if (!dev_mount)
+                return 0;
+
+        if (!dev_mnt)
+                return 0;
+
+        err = kern_path(mountpoint, LOOKUP_FOLLOW, &path);
+        if (err)
+                return err;
+        err = do_add_mount(dev_mnt, &path, 0, NULL);
+        if (err)
+                printk(KERN_INFO "devtmpfs: error mounting %i\n", err);
+        else
+                printk(KERN_INFO "devtmpfs: mounted\n");
+        path_put(&path);
+        return err;
+}
+
+/*
+ * Create devtmpfs instance, driver-core devices will add their device
+ * nodes here.
+ */
+int __init devtmpfs_init(void)
+{
+        int err;
+        struct vfsmount *mnt;
+
+        err = register_filesystem(&dev_fs_type);
+        if (err) {
+                printk(KERN_ERR "devtmpfs: unable to register devtmpfs "
+                       "type %i\n", err);
+                return err;
+        }
+
+        mnt = kern_mount(&dev_fs_type);
+        if (IS_ERR(mnt)) {
+                err = PTR_ERR(mnt);
+                printk(KERN_ERR "devtmpfs: unable to create devtmpfs %i\n", err);
+                unregister_filesystem(&dev_fs_type);
+                return err;
+        }
+        dev_mnt = mnt;
+
+        printk(KERN_INFO "devtmpfs: initialized\n");
+        return 0;
+}