[Devel] Re: [PATCH 4/4] The control group itself

[Pavel Emelyanov]

Paul Menage wrote:
> On Jan 8, 2008 1:18 AM, Pavel Emelyanov <xemul at openvz.org> wrote:
>> Each new group will have its own maps for char and block
>> layers. The devices access list is tuned via the
>> devices.permissions file.
>>
>> One may read from the file to get the configured
>> state. E.g.
>>
>> # cat /cont/devices/0/devices.permissions
>> c 1:* rw
>> b 8:1 rw
>>
>> The top container isn't initialized, so that the char
>> and block layers will use the global maps to lookup
>> their devices.
>>
>> Signed-off-by: Pavel Emelyanov <xemul at openvz.org>
>>
>> ---
>>
>> diff --git a/fs/Makefile b/fs/Makefile
>> index 82b6ae1..a085706 100644
>> --- a/fs/Makefile
>> +++ b/fs/Makefile
>> @@ -63,6 +63,8 @@ obj-y                         += devpts/
>>
>>  obj-$(CONFIG_PROFILING)                += dcookies.o
>>  obj-$(CONFIG_DLM)              += dlm/
>> +
>> +obj-$(CONFIG_CGROUP_DEVS)      += devscontrol.o
>>
>>  # Do not add any filesystems before this line
>>  obj-$(CONFIG_REISERFS_FS)      += reiserfs/
>> diff --git a/fs/devscontrol.c b/fs/devscontrol.c
>> new file mode 100644
>> index 0000000..ea282f3
>> --- /dev/null
>> +++ b/fs/devscontrol.c
>> @@ -0,0 +1,291 @@
>> +/*
>> + * devscontrol.c - Device Controller
>> + *
>> + * Copyright 2007 OpenVZ SWsoft Inc
>> + * Author: Pavel Emelyanov <xemul at openvz.org>
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License as published by
>> + * the Free Software Foundation; either version 2 of the License, or
>> + * (at your option) any later version.
>> + *
>> + * This program is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> + * GNU General Public License for more details.
>> + */
>> +
>> +#include <linux/cgroup.h>
>> +#include <linux/cdev.h>
>> +#include <linux/err.h>
>> +#include <linux/devscontrol.h>
>> +#include <linux/uaccess.h>
>> +#include <linux/fs.h>
>> +#include <linux/genhd.h>
>> +
>> +struct devs_cgroup {
>> +       struct cgroup_subsys_state css;
>> +
>> +       struct kobj_map *cdev_map;
>> +       struct kobj_map *bdev_map;
>> +};
>> +
>> +static inline
>> +struct devs_cgroup *css_to_devs(struct cgroup_subsys_state *css)
>> +{
>> +       return container_of(css, struct devs_cgroup, css);
>> +}
>> +
>> +static inline
>> +struct devs_cgroup *cgroup_to_devs(struct cgroup *cont)
>> +{
>> +       return css_to_devs(cgroup_subsys_state(cont, devs_subsys_id));
>> +}
>> +
>> +struct kobj_map *task_cdev_map(struct task_struct *tsk)
>> +{
>> +       struct cgroup_subsys_state *css;
>> +
>> +       css = task_subsys_state(tsk, devs_subsys_id);
>> +       if (css->cgroup->parent == NULL)
>> +               return NULL;
>> +       else
>> +               return css_to_devs(css)->cdev_map;
>> +}
> 
> For this (and task_bdev_map) it would be cleaner to just leave the
> cdev_map and bdev_map in the root cgroup as NULL, so you could just
> make this
> 
> struct kobj_map *task_cdev_map(struct task_struct *tsk)
> {
>   return css_to_devs(task_subsys_state(tsk, devs_subsys_id))->cdev_map;
> }

Ok, thanks, I'll make it in the next version.

>> +
>> +static ssize_t devs_write(struct cgroup *cont, struct cftype *cft,
>> +               struct file *f, const char __user *ubuf,
>> +               size_t nbytes, loff_t *pos)
>> +{
>> +       int err, all, chrdev;
>> +       dev_t dev;
>> +       char buf[64];
>> +       struct devs_cgroup *devs;
>> +       mode_t mode;
>> +
>> +       if (copy_from_user(buf, ubuf, sizeof(buf)))
>> +               return -EFAULT;
>> +
>> +       buf[sizeof(buf) - 1] = 0;
>> +       err = decode_perms_str(buf, &chrdev, &dev, &all, &mode);
>> +       if (err < 0)
>> +               return err;
>> +
>> +       devs = cgroup_to_devs(cont);
>> +
>> +       if (mode == 0) {
>> +               if (chrdev)
>> +                       err = cdev_del_from_map(devs->cdev_map, dev, all);
>> +               else
>> +                       err = bdev_del_from_map(devs->bdev_map, dev, all);
> 
> There's no locking involved on these calls, other than the cgroups
> code guaranteeing that the subsystem objects themselves won't go away.
> A quick look over the kobj_map calls suggests that these calls may be
> thread-safe - can you confirm that. (And maybe comment at the top of
> the function?)

Yup. The locking inside this call is unnecessary as maps
are atomic by themselves. I'll add the comment about it.

>> +       arg.buf = (char *)__get_free_page(GFP_KERNEL);
>> +       if (arg.buf == NULL)
>> +               return -ENOMEM;
>> +
>> +       devs = cgroup_to_devs(cont);
>> +       arg.pos = 0;
>> +
>> +       arg.chrdev = 1;
>> +       cdev_iterate_map(devs->cdev_map, devs_dump_one, &arg);
>> +
>> +       arg.chrdev = 0;
>> +       bdev_iterate_map(devs->bdev_map, devs_dump_one, &arg);
> 
> Is there any chance of this overflowing the buffer page?

Well, I have checks that we've filled all the page and do not
go on dumping the info in this case. So the only bad thing user
can have is that he doesn't see the full perms list, but the
kernel won't OOPs ;) Since we have from 8 to 16 bytes per perm
line, we may have from 256 to 256 permissions set for cgroup. 

Not that much, but that's enough for a first time (I haven't seen 
any OpenVZ user who sets more than 50 devperms). But sure this 
is one of TODO-s for the next patch versions.

> Paul
> 

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