Re: [PATCH] cgroups: implement device whitelist (v4)

Serge E. Hallyn
2008-03-18T14:11:08Z

Quoting Li Zefan (lizf@cn.fujitsu.com):
> Serge E. Hallyn wrote:
> > Implement a cgroup to track and enforce open and mknod restrictions on device
> > files. A device cgroup associates a device access whitelist with each
> > cgroup. A whitelist entry has 4 fields. 'type' is a (all), c (char), or
> > b (block). 'all' means it applies to all types and all major and minor
> > numbers. Major and minor are either an integer or * for all.
> > Access is a composition of r (read), w (write), and m (mknod).
> >
> > The root device cgroup starts with rwm to 'all'. A child devcg gets
> > a copy of the parent. Admins can then remove devices from the
> > whitelist or add new entries. A child cgroup can never receive a
> > device access which is denied its parent. However when a device
> > access is removed from a parent it will not also be removed from the
> > child(ren).
> >
> > An entry is added using devices.allow, and removed using
> > devices.deny. For instance
> >
> > echo 'c 1:3 mr' > /cgroups/1/devices.allow
> >
> > allows cgroup 1 to read and mknod the device usually known as
> > /dev/null. Doing
> >
> > echo a > /cgroups/1/devices.deny
> >
> > will remove the default 'a *:* mrw' entry.
> >
> > CAP_SYS_ADMIN is needed to change permissions or move another task
> > to a new cgroup. A cgroup may not be granted more permissions than
> > the cgroup's parent has. Any task can move itself between cgroups.
> > This won't be sufficient, but we can decide the best way to
> > adequately restrict movement later.
> >
> > The parsing of devices.allow/deny needs to be cleaned up a bit and
> > Documented. I'd like to get an idea whether this approach is otherwise
> > acceptable.
> >
> > Changelog:
> > Mar 17 2008: Place specific device cgroup hooks next to
> > security_inode_{mknod,permission} rather than using
> > the security hooks.
> > Also remove most of the controls over tasks moving
> > between cgroups and playing with the allow and deny
> > permissions.
> > Switch to major:minor format.
> > Rename devcg to 'devices' to conform to cgroup naming.
> > Mar 13 2008: move the dev_cgroup support into
> > capability hooks instead of having it
> > as a separate security module.
> > Support root_plug with devcg.
> > Note that due to this change, devcg will
> > not be enforcing if the dummy module is
> > loaded, or if selinux is loaded without
> > capabilities.
> > Mar 12 2008: allow dev_cgroup lsm to be used when
> > SECURITY=n, and allow stacking with SELinux
> > and Smack. Don't work too hard in Kconfig
> > to prevent a warning when smack+devcg are
> > both compiled in, worry about that later.
> >
>
> I would like to give some comments in the code. :)

Thanks for taking a look.

> > Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
> > ---
> > fs/namei.c | 9 +
> > include/linux/cgroup_subsys.h | 6 +
> > include/linux/device_cgroup.h | 12 +
> > init/Kconfig | 7 +
> > security/Makefile | 1 +
> > security/device_cgroup.c | 597 +++++++++++++++++++++++++++++++++++++++++
> > 6 files changed, 632 insertions(+), 0 deletions(-)
> > create mode 100644 include/linux/device_cgroup.h
> > create mode 100644 security/device_cgroup.c
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index dfb3cb8..6caed32 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -30,6 +30,7 @@
> > #include <linux/capability.h>
> > #include <linux/file.h>
> > #include <linux/fcntl.h>
> > +#include <linux/device_cgroup.h>
> > #include <asm/namei.h>
> > #include <asm/uaccess.h>
> >
> > @@ -281,6 +282,10 @@ int permission(struct inode *inode, int mask, struct nameidata *nd)
> > if (retval)
> > return retval;
> >
> > + retval = devcgroup_inode_permission(inode, mask);
> > + if (retval)
> > + return retval;
> > +
> > return security_inode_permission(inode, mask, nd);
> > }
> >
> > @@ -2028,6 +2033,10 @@ int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
> > if (!dir->i_op || !dir->i_op->mknod)
> > return -EPERM;
> >
> > + error = devcgroup_inode_mknod(mode, dev);
> > + if (error)
> > + return error;
> > +
> > error = security_inode_mknod(dir, dentry, mode, dev);
> > if (error)
> > return error;
> > diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
> > index 1ddebfc..e287745 100644
> > --- a/include/linux/cgroup_subsys.h
> > +++ b/include/linux/cgroup_subsys.h
> > @@ -42,3 +42,9 @@ SUBSYS(mem_cgroup)
> > #endif
> >
> > /* */
> > +
> > +#ifdef CONFIG_CGROUP_DEVICE
> > +SUBSYS(devices)
> > +#endif
> > +
> > +/* */
> > diff --git a/include/linux/device_cgroup.h b/include/linux/device_cgroup.h
> > new file mode 100644
> > index 0000000..0b0d9c3
> > --- /dev/null
> > +++ b/include/linux/device_cgroup.h
> > @@ -0,0 +1,12 @@
> > +#include <linux/module.h>
> > +#include <linux/fs.h>
> > +
> > +#ifdef CONFIG_CGROUP_DEVICE
> > +extern int devcgroup_inode_permission(struct inode *inode, int mask);
> > +extern int devcgroup_inode_mknod(int mode, dev_t dev);
> > +#else
> > +static inline int devcgroup_inode_permission(struct inode *inode, int mask)
> > +{ return 0; }
> > +static inline int devcgroup_inode_mknod(int mode, dev_t dev)
> > +{ return 0; }
> > +#endif
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 009f2d8..30868cd 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -298,6 +298,13 @@ config CGROUP_NS
> > for instance virtual servers and checkpoint/restart
> > jobs.
> >
> > +config CGROUP_DEVICE
> > + bool "Device controller for cgroups"
> > + depends on CGROUPS && EXPERIMENTAL
> > + help
> > + Provides a cgroup implementing whitelists for devices which
> > + a process in the cgroup can mknod or open.
> > +
> > config CPUSETS
> > bool "Cpuset support"
> > depends on SMP && CGROUPS
> > diff --git a/security/Makefile b/security/Makefile
> > index 9e8b025..7ef1107 100644
> > --- a/security/Makefile
> > +++ b/security/Makefile
> > @@ -18,3 +18,4 @@ obj-$(CONFIG_SECURITY_SELINUX) += selinux/built-in.o
> > obj-$(CONFIG_SECURITY_SMACK) += commoncap.o smack/built-in.o
> > obj-$(CONFIG_SECURITY_CAPABILITIES) += commoncap.o capability.o
> > obj-$(CONFIG_SECURITY_ROOTPLUG) += commoncap.o root_plug.o
> > +obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
> > diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> > new file mode 100644
> > index 0000000..33d8fd8
> > --- /dev/null
> > +++ b/security/device_cgroup.c
> > @@ -0,0 +1,597 @@
> > +/*
> > + * dev_cgroup.c - device cgroup subsystem
> > + *
> > + * Copyright 2007 IBM Corp
> > + */
> > +
> > +#include <linux/device_cgroup.h>
> > +#include <linux/cgroup.h>
> > +#include <linux/ctype.h>
> > +#include <linux/list.h>
> > +#include <asm/uaccess.h>
> > +
> > +#define ACC_MKNOD 1
> > +#define ACC_READ 2
> > +#define ACC_WRITE 4
> > +#define ACC_MASK (ACC_MKNOD | ACC_READ | ACC_WRITE)
> > +
> > +#define DEV_BLOCK 1
> > +#define DEV_CHAR 2
> > +#define DEV_ALL 4 /* this represents all devices */
> > +
> > +/*
> > + * whitelist locking rules:
> > + * cgroup_lock() cannot be taken under cgroup->lock.
>
> When you say cgroup->lock, you mean dev_cgroup->lock, right?
> So would it be better to make it clear in the comment?

Yes.

> > + * cgroup->lock can be taken with or without cgroup_lock().
> > + *
> > + * modifications always require cgroup_lock
> > + * modifications to a list which is visible require the
> > + * cgroup->lock *and* cgroup_lock()
> > + * walking the list requires cgroup->lock or cgroup_lock().
> > + *
> > + * reasoning: dev_whitelist_copy() needs to kmalloc, so needs
> > + * a mutex, which the cgroup_lock() is. Since modifying
> > + * a visible list requires both locks, either lock can be
> > + * taken for walking the list. Since the wh->spinlock is taken
> > + * for modifying a public-accessible list, the spinlock is
> > + * sufficient for just walking the list.
> > + */
> > +
> > +struct dev_whitelist_item {
> > + u32 major, minor;
> > + short type;
> > + short access;
> > + struct list_head list;
> > +};
> > +
> > +struct dev_cgroup {
> > + struct cgroup_subsys_state css;
> > + struct list_head whitelist;
> > + spinlock_t lock;
> > +};
> > +
> > +static inline struct dev_cgroup *cgroup_to_devcgroup(
> > + struct cgroup *cgroup)
> > +{
> > + return container_of(cgroup_subsys_state(cgroup, devices_subsys_id),
> > + struct dev_cgroup, css);
> > +}
> > +
> > +
> > +struct cgroup_subsys devices_subsys;
> > +
> > +static int devcgroup_can_attach(struct cgroup_subsys *ss,
> > + struct cgroup *new_cgroup, struct task_struct *task)
> > +{
> > +
>
> redundant empty line
>
> > + if (current != task && !capable(CAP_SYS_ADMIN))
> > + return -EPERM;
> > +
> > + return 0;
> > +}
> > +
> > +/*
> > + * called under cgroup_lock()
> > + */
> > +int dev_whitelist_copy(struct list_head *dest, struct list_head *orig)
>
> static int
>
> > +{
> > + struct dev_whitelist_item *wh, *tmp, *new;
> > +
> > + list_for_each_entry(wh, orig, list) {
> > + new = kmalloc(sizeof(*wh), GFP_KERNEL);
> > + if (!new)
> > + goto free_and_exit;
> > + new->major = wh->major;
> > + new->minor = wh->minor;
> > + new->type = wh->type;
> > + new->access = wh->access;
> > + list_add_tail(&new->list, dest);
> > + }
> > +
> > + return 0;
> > +
> > +free_and_exit:
> > + list_for_each_entry_safe(wh, tmp, dest, list) {
> > + list_del(&wh->list);
> > + kfree(wh);
> > + }
> > + return -ENOMEM;
> > +}
> > +
> > +/* Stupid prototype - don't bother combining existing entries */
> > +/*
> > + * called under cgroup_lock()
> > + * since the list is visible to dev_whitelist_addother tasks, we need the spinlock also
> > + */
> > +int dev_whitelist_add(struct dev_cgroup *dev_cgroup,
> > + struct dev_whitelist_item *wh)
>
> ditoo
>
> > +{
> > + struct dev_whitelist_item *whcopy;
> > +
> > + whcopy = kmalloc(sizeof(*whcopy), GFP_KERNEL);
> > + if (!whcopy)
> > + return -ENOMEM;
> > +
> > + memcpy(whcopy, wh, sizeof(*whcopy));
> > + spin_lock(&dev_cgroup->lock);
> > + list_add_tail(&whcopy->list, &dev_cgroup->whitelist);
> > + spin_unlock(&dev_cgroup->lock);
> > + return 0;
> > +}
> > +
> > +/*
> > + * called under cgroup_lock()
> > + * since the list is visible to other tasks, we need the spinlock also
> > + */
> > +void dev_whitelist_rm(struct dev_cgroup *dev_cgroup,
> > + struct dev_whitelist_item *wh)
>
> ditto
>
> > +{
> > + struct dev_whitelist_item *walk, *tmp;
> > +
> > + spin_lock(&dev_cgroup->lock);
> > + list_for_each_entry_safe(walk, tmp, &dev_cgroup->whitelist, list) {
> > + if (walk->type == DEV_ALL)
> > + goto remove;
> > + if (walk->type != wh->type)
> > + continue;
> > + if (walk->major != ~0 && walk->major != wh->major)
> > + continue;
> > + if (walk->minor != ~0 && walk->minor != wh->minor)
> > + continue;
> > +
> > +remove:
> > + walk->access &= ~wh->access;
> > + if (!walk->access) {
> > + list_del(&walk->list);
> > + kfree(walk);
> > + }
> > + }
> > + spin_unlock(&dev_cgroup->lock);
> > +}
> > +
> > +/*
> > + * called from kernel/cgroup.c with cgroup_lock() held.
> > + */
> > +static struct cgroup_subsys_state *devcgroup_create(struct cgroup_subsys *ss,
> > + struct cgroup *cgroup)
> > +{
> > + struct dev_cgroup *dev_cgroup, *parent_dev_cgroup;
> > + struct cgroup *parent_cgroup;
> > + int ret;
> > +
> > + dev_cgroup = kzalloc(sizeof(*dev_cgroup), GFP_KERNEL);
> > + if (!dev_cgroup)
> > + return ERR_PTR(-ENOMEM);
> > + INIT_LIST_HEAD(&dev_cgroup->whitelist);
> > + parent_cgroup = cgroup->parent;
> > +
> > + if (parent_cgroup == NULL) {
> > + struct dev_whitelist_item *wh;
> > + wh = kmalloc(sizeof(*wh), GFP_KERNEL);
>
> if (wh == NULL) ..

Egads. Over the weekend I had switched these to using a statically
defined init_dev_cgroup and init_wh, but switched that back. Yes
definately needs a check.

> > + wh->minor = wh->major = ~0;
> > + wh->type = DEV_ALL;
> > + wh->access = ACC_MKNOD | ACC_READ | ACC_WRITE;
> > + list_add(&wh->list, &dev_cgroup->whitelist);
> > + } else {
> > + parent_dev_cgroup = cgroup_to_devcgroup(parent_cgroup);
> > + ret = dev_whitelist_copy(&dev_cgroup->whitelist,
> > + &parent_dev_cgroup->whitelist);
> > + if (ret) {
> > + kfree(dev_cgroup);
> > + return ERR_PTR(ret);
> > + }
> > + }
> > +
> > + spin_lock_init(&dev_cgroup->lock);
> > + return &dev_cgroup->css;
> > +}
> > +
> > +static void devcgroup_destroy(struct cgroup_subsys *ss,
> > + struct cgroup *cgroup)
> > +{
> > + struct dev_cgroup *dev_cgroup;
> > + struct dev_whitelist_item *wh, *tmp;
> > +
> > + dev_cgroup = cgroup_to_devcgroup(cgroup);
> > + list_for_each_entry_safe(wh, tmp, &dev_cgroup->whitelist, list) {
> > + list_del(&wh->list);
> > + kfree(wh);
> > + }
> > + kfree(dev_cgroup);
> > +}
> > +
> > +#define DEVCG_ALLOW 1
> > +#define DEVCG_DENY 2
> > +
> > +void set_access(char *acc, short access)
>
> static
>
> > +{
> > + int idx = 0;
> > + memset(acc, 0, 4);
> > + if (access & ACC_READ)
> > + acc[idx++] = 'r';
> > + if (access & ACC_WRITE)
> > + acc[idx++] = 'w';
> > + if (access & ACC_MKNOD)
> > + acc[idx++] = 'm';
> > +}
> > +
> > +char type_to_char(short type)
>
> static
>
> > +{
> > + if (type == DEV_ALL)
> > + return 'a';
> > + if (type == DEV_CHAR)
> > + return 'c';
> > + if (type == DEV_BLOCK)
> > + return 'b';
> > + return 'X';
> > +}
> > +
> > +static void set_majmin(char *str, int len, unsigned m)
> > +{
> > + memset(str, 0, len);
> > + if (m == ~0)
> > + sprintf(str, "*");
> > + else
> > + snprintf(str, len, "%d", m);
> > +}
> > +
> > +char *print_whitelist(struct dev_cgroup *devcgroup, int *len)
>
> static
>
> > +{
> > + char *buf, *s, acc[4];
> > + struct dev_whitelist_item *wh;
> > + int ret;
> > + int count = 0;
> > + char maj[10], min[10];
> > +
> > + buf = kmalloc(4096, GFP_KERNEL);
> > + if (!buf)
> > + return ERR_PTR(-ENOMEM);
> > + s = buf;
> > + *s = '\0';
> > + *len = 0;
> > +
> > + spin_lock(&devcgroup->lock);
> > + list_for_each_entry(wh, &devcgroup->whitelist, list) {
> > + set_access(acc, wh->access);
> > + set_majmin(maj, 10, wh->major);
> > + set_majmin(min, 10, wh->minor);
> > + ret = snprintf(s, 4095-(s-buf), "%c %s:%s %s\n",
> > + type_to_char(wh->type), maj, min, acc);
> > + if (s+ret >= buf+4095) {
> > + kfree(buf);
> > + buf = ERR_PTR(-ENOMEM);
> > + break;
> > + }
> > + s += ret;
> > + *len += ret;
> > + count++;
> > + }
> > + spin_unlock(&devcgroup->lock);
> > +
> > + return buf;
> > +}
> > +
> > +static ssize_t devcgroup_access_read(struct cgroup *cgroup,
> > + struct cftype *cft, struct file *file,
> > + char __user *userbuf, size_t nbytes, loff_t *ppos)
> > +{
> > + struct dev_cgroup *devcgroup = cgroup_to_devcgroup(cgroup);
> > + int filetype = cft->private;
> > + char *buffer;
> > + int len, retval;
> > +
> > + if (filetype != DEVCG_ALLOW)
> > + return -EINVAL;
> > + buffer = print_whitelist(devcgroup, &len);
> > + if (IS_ERR(buffer))
> > + return PTR_ERR(buffer);
> > +
> > + retval = simple_read_from_buffer(userbuf, nbytes, ppos, buffer, len);
> > + kfree(buffer);
> > + return retval;
> > +}
> > +
> > +/*
> > + * may_access_whitelist:
> > + * does the access granted to dev_cgroup c contain the access
> > + * requested in whitelist item refwh.
> > + * return 1 if yes, 0 if no.
> > + * call with c->lock held
> > + */
> > +static int may_access_whitelist(struct dev_cgroup *c,
> > + struct dev_whitelist_item *refwh)
> > +{
> > + struct dev_whitelist_item *whitem;
> > +
> > + list_for_each_entry(whitem, &c->whitelist, list) {
> > + if (whitem->type & DEV_ALL)
> > + return 1;
> > + if ((refwh->type & DEV_BLOCK) && !(whitem->type & DEV_BLOCK))
> > + continue;
> > + if ((refwh->type & DEV_CHAR) && !(whitem->type & DEV_CHAR))
> > + continue;
> > + if (whitem->major != ~0 && whitem->major != refwh->major)
> > + continue;
> > + if (whitem->minor != ~0 && whitem->minor != refwh->minor)
> > + continue;
> > + if (refwh->access & (~(whitem->access | ACC_MASK)))
> > + continue;
> > + return 1;
> > + }
> > + return 0;
> > +}
> > +
> > +/*
> > + * parent_has_perm:
> > + * when adding a new allow rule to a device whitelist, the rule
> > + * must be allowed in the parent device
> > + */
> > +static int parent_has_perm(struct cgroup *childcg,
> > + struct dev_whitelist_item *wh)
> > +{
> > + struct cgroup *pcg = childcg->parent;
> > + struct dev_cgroup *parent;
> > + int ret;
> > +
> > + if (!pcg)
> > + return 1;
> > + parent = cgroup_to_devcgroup(pcg);
> > + spin_lock(&parent->lock);
> > + ret = may_access_whitelist(parent, wh);
> > + spin_unlock(&parent->lock);
> > + return ret;
> > +}
> > +
> > +/*
> > + * Modify the whitelist using allow/deny rules.
> > + * CAP_SYS_ADMIN is needed for this. It's at least separate from CAP_MKNOD
> > + * so we can give a container CAP_MKNOD to let it create devices but not
> > + * modify the whitelist.
> > + * It seems likely we'll want to add a CAP_CONTAINER capability to allow
> > + * us to also grant CAP_SYS_ADMIN to containers without giving away the
> > + * device whitelist controls, but for now we'll stick with CAP_SYS_ADMIN
> > + *
> > + * Taking rules away is always allowed (given CAP_SYS_ADMIN). Granting
> > + * new access is only allowed if you're in the top-level cgroup, or your
> > + * parent cgroup has the access you're asking for.
> > + */
> > +static ssize_t devcgroup_access_write(struct cgroup *cgroup, struct cftype *cft,
> > + struct file *file, const char __user *userbuf,
> > + size_t nbytes, loff_t *ppos)
> > +{
> > + struct cgroup *cur_cgroup;
> > + struct dev_cgroup *devcgroup, *cur_devcgroup;
> > + int filetype = cft->private;
> > + char *buffer, *b;
> > + int retval = 0, count;
> > + struct dev_whitelist_item wh;
> > +
> > + if (!capable(CAP_SYS_ADMIN))
> > + return -EPERM;
> > +
> > + devcgroup = cgroup_to_devcgroup(cgroup);
> > + cur_cgroup = task_cgroup(current, devices_subsys.subsys_id);
> > + cur_devcgroup = cgroup_to_devcgroup(cur_cgroup);
> > +
> > + buffer = kmalloc(nbytes+1, GFP_KERNEL);
> > + if (!buffer)
> > + return -ENOMEM;
> > +
> > + if (copy_from_user(buffer, userbuf, nbytes)) {
> > + retval = -EFAULT;
> > + goto out1;
> > + }
> > + buffer[nbytes] = 0; /* nul-terminate */
> > +
> > + cgroup_lock();
> > + if (cgroup_is_removed(cgroup)) {
> > + retval = -ENODEV;
> > + goto out2;
> > + }
> > +
> > + memset(&wh, 0, sizeof(wh));
> > + b = buffer;
> > +
> > + switch (*b) {
> > + case 'a':
> > + wh.type = DEV_ALL;
> > + wh.access = ACC_MASK;
> > + goto handle;
> > + case 'b':
> > + wh.type = DEV_BLOCK;
> > + break;
> > + case 'c':
> > + wh.type = DEV_CHAR;
> > + break;
> > + default:
> > + retval = -EINVAL;
> > + goto out2;
> > + }
> > + b += 2;
> > + if (*b == '*') {
> > + wh.major = ~0;
> > + b++;
> > + } else if (isdigit(*b)) {
> > + wh.major = 0;
> > + while (isdigit(*b)) {
> > + wh.major = wh.major*10+(*b-'0');
> > + b++;
> > + }
> > + } else {
> > + retval = -EINVAL;
> > + goto out2;
> > + }
> > + if (*b != ':') {
> > + retval = -EINVAL;
> > + goto out2;
> > + }
> > + b++;
> > +
> > + /* read minor */
> > + if (*b == '*') {
> > + wh.minor = ~0;
> > + b++;
> > + } else if (isdigit(*b)) {
> > + wh.minor = 0;
> > + while (isdigit(*b)) {
> > + wh.minor = wh.minor*10+(*b-'0');
> > + b++;
> > + }
> > + } else {
> > + retval = -EINVAL;
> > + goto out2;
> > + }
> > + if (!isspace(*b)) {
> > + retval = -EINVAL;
> > + goto out2;
> > + }
> > + for (b++, count = 0; count < 3; count++, b++) {
> > + switch (*b) {
> > + case 'r':
> > + wh.access |= ACC_READ;
> > + break;
> > + case 'w':
> > + wh.access |= ACC_WRITE;
> > + break;
> > + case 'm':
> > + wh.access |= ACC_MKNOD;
> > + break;
> > + case '\n':
> > + case '\0':
> > + break;
> > + default:
> > + retval = -EINVAL;
> > + goto out2;
> > + }
> > + }
> > +
> > +handle:
> > + retval = 0;
> > + switch (filetype) {
> > + case DEVCG_ALLOW:
> > + if (!parent_has_perm(cgroup, &wh))
> > + retval = -EPERM;
> > + else
> > + retval = dev_whitelist_add(devcgroup, &wh);
> > + break;
> > + case DEVCG_DENY:
> > + dev_whitelist_rm(devcgroup, &wh);
> > + break;
> > + default:
> > + retval = -EINVAL;
> > + goto out2;
> > + }
> > +
> > + if (retval == 0)
> > + retval = nbytes;
> > +
> > +out2:
> > + cgroup_unlock();
> > +out1:
> > + kfree(buffer);
> > + return retval;
> > +}
> > +
> > +static struct cftype dev_cgroup_files[] = {
> > + {
> > + .name = "allow",
> > + .read = devcgroup_access_read,
> > + .write = devcgroup_access_write,
> > + .private = DEVCG_ALLOW,
> > + },
> > + {
> > + .name = "deny",
> > + .write = devcgroup_access_write,
> > + .private = DEVCG_DENY,
> > + },
> > +};
> > +
> > +static int devcgroup_populate(struct cgroup_subsys *ss,
> > + struct cgroup *cont)
>
> could you use the name 'cgroup' or 'cgrp' instead of old name 'cont'?

Sure. Certainly not cgrp, will use cgroup.

> > + .name = "devices",
> > + .can_attach = devcgroup_can_attach,
> > + .create = devcgroup_create,
> > + .destroy = devcgroup_destroy,
> > + .populate = devcgroup_populate,
> > + .subsys_id = devices_subsys_id,
> > +};

thanks,
-serge