Quoting menage@google.com (menage@google.com):
> This patch converts devcgroup_access_write() from a raw file handler
> into a handler for the cgroup write_string() method. This allows some
> boilerplate copying/locking/checking to be removed and simplifies the
> cleanup path, since these functions are performed by the cgroups
> framework before calling the handler.
> 
> Signed-off-by: Paul Menage <menage@google.com>

Looks good.  I'll have to test later.

Acked-by: Serge Hallyn <serue@us.ibm.com>

thanks,
-serge

> 
> ---
>  security/device_cgroup.c |  103 +++++++++++++++++------------------------------
>  1 file changed, 39 insertions(+), 64 deletions(-)
> 
> Index: cws-2.6.26-rc5-mm3/security/device_cgroup.c
> ===================================================================
> --- cws-2.6.26-rc5-mm3.orig/security/device_cgroup.c
> +++ cws-2.6.26-rc5-mm3/security/device_cgroup.c
> @@ -59,6 +59,11 @@ static inline struct dev_cgroup *cgroup_
>  	return css_to_devcgroup(cgroup_subsys_state(cgroup, devices_subsys_id));
>  }
> 
> +static inline struct dev_cgroup *task_devcgroup(struct task_struct *task)
> +{
> +	return css_to_devcgroup(task_subsys_state(task, devices_subsys_id));
> +}
> +
>  struct cgroup_subsys devices_subsys;
> 
>  static int devcgroup_can_attach(struct cgroup_subsys *ss,
> @@ -312,10 +317,10 @@ static int may_access_whitelist(struct d
>   * 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,
> +static int parent_has_perm(struct dev_cgroup *childcg,
>  				  struct dev_whitelist_item *wh)
>  {
> -	struct cgroup *pcg = childcg->parent;
> +	struct cgroup *pcg = childcg->css.cgroup->parent;
>  	struct dev_cgroup *parent;
>  	int ret;
> 
> @@ -341,39 +346,18 @@ static int parent_has_perm(struct cgroup
>   * 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;
> +static int devcgroup_update_access(struct dev_cgroup *devcgroup,
> +				   int filetype, const char *buffer)
> +{
> +	struct dev_cgroup *cur_devcgroup;
> +	const char *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;
> -	}
> +	cur_devcgroup = task_devcgroup(current);
> 
>  	memset(&wh, 0, sizeof(wh));
>  	b = buffer;
> @@ -390,14 +374,11 @@ static ssize_t devcgroup_access_write(st
>  		wh.type = DEV_CHAR;
>  		break;
>  	default:
> -		retval = -EINVAL;
> -		goto out2;
> +		return -EINVAL;
>  	}
>  	b++;
> -	if (!isspace(*b)) {
> -		retval = -EINVAL;
> -		goto out2;
> -	}
> +	if (!isspace(*b))
> +		return -EINVAL;
>  	b++;
>  	if (*b == '*') {
>  		wh.major = ~0;
> @@ -409,13 +390,10 @@ static ssize_t devcgroup_access_write(st
>  			b++;
>  		}
>  	} else {
> -		retval = -EINVAL;
> -		goto out2;
> -	}
> -	if (*b != ':') {
> -		retval = -EINVAL;
> -		goto out2;
> +		return -EINVAL;
>  	}
> +	if (*b != ':')
> +		return -EINVAL;
>  	b++;
> 
>  	/* read minor */
> @@ -429,13 +407,10 @@ static ssize_t devcgroup_access_write(st
>  			b++;
>  		}
>  	} else {
> -		retval = -EINVAL;
> -		goto out2;
> -	}
> -	if (!isspace(*b)) {
> -		retval = -EINVAL;
> -		goto out2;
> +		return -EINVAL;
>  	}
> +	if (!isspace(*b))
> +		return -EINVAL;
>  	for (b++, count = 0; count < 3; count++, b++) {
>  		switch (*b) {
>  		case 'r':
> @@ -452,8 +427,7 @@ static ssize_t devcgroup_access_write(st
>  			count = 3;
>  			break;
>  		default:
> -			retval = -EINVAL;
> -			goto out2;
> +			return -EINVAL;
>  		}
>  	}
> 
> @@ -461,38 +435,39 @@ handle:
>  	retval = 0;
>  	switch (filetype) {
>  	case DEVCG_ALLOW:
> -		if (!parent_has_perm(cgroup, &wh))
> -			retval = -EPERM;
> -		else
> -			retval = dev_whitelist_add(devcgroup, &wh);
> -		break;
> +		if (!parent_has_perm(devcgroup, &wh))
> +			return -EPERM;
> +		return dev_whitelist_add(devcgroup, &wh);
>  	case DEVCG_DENY:
>  		dev_whitelist_rm(devcgroup, &wh);
>  		break;
>  	default:
> -		retval = -EINVAL;
> -		goto out2;
> +		return -EINVAL;
>  	}
> +	return 0;
> +}
> 
> -	if (retval == 0)
> -		retval = nbytes;
> -
> -out2:
> +static int devcgroup_access_write(struct cgroup *cgrp, struct cftype *cft,
> +				  const char *buffer)
> +{
> +	int retval;
> +	if (!cgroup_lock_live_group(cgrp))
> +		return -ENODEV;
> +	retval = devcgroup_update_access(cgroup_to_devcgroup(cgrp),
> +					 cft->private, buffer);
>  	cgroup_unlock();
> -out1:
> -	kfree(buffer);
>  	return retval;
>  }
> 
>  static struct cftype dev_cgroup_files[] = {
>  	{
>  		.name = "allow",
> -		.write  = devcgroup_access_write,
> +		.write_string  = devcgroup_access_write,
>  		.private = DEVCG_ALLOW,
>  	},
>  	{
>  		.name = "deny",
> -		.write = devcgroup_access_write,
> +		.write_string = devcgroup_access_write,
>  		.private = DEVCG_DENY,
>  	},
>  	{
> 
> --
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