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


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