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

[Pavel Emelyanov]

sukadev at us.ibm.com wrote:
> This patchset does fix the problem I was having before with null and
> zero devices. Overall, it looks like pretty good.
> 
> I am still reviewing the patches.  Just some nits I came across:
> 
> 
> Pavel Emelianov [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.
> | 
> | The top container isn't initialized, so that the char 
> | and block layers will use the global maps to lookup 
> | their devices. I did that not to export the static maps
> | to the outer world.
> | 
> | Good news is that this patch now contains more comments 
> | and Documentation file :)
> | 
> | Signed-off-by: Pavel Emelyanov <xemul at openvz.org>
> | 
> | ---
> | 
> | diff --git a/Documentation/controllers/devices.txt b/Documentation/controllers/devices.txt
> | new file mode 100644
> | index 0000000..dbd0c7a
> | --- /dev/null
> | +++ b/Documentation/controllers/devices.txt
> | @@ -0,0 +1,61 @@
> | +
> | +	Devices visibility controller
> | +
> | +This controller allows to tune the devices accessibility by tasks,
> | +i.e. grant full access for /dev/null, /dev/zero etc, grant read-only
> | +access to IDE devices and completely hide SCSI disks.
> | +
> | +Tasks still can call mknod to create device files, regardless of
> | +whether the particular device is visible or accessible, but they
> | +may not be able to open it later.
> | +
> | +This one hides under CONFIG_CGROUP_DEVS option.
> | +
> | +
> | +Configuring
> | +
> | +The controller provides a single file to configure itself -- the
> | +devices.permissions one. To change the accessibility level for some
> | +device write the following string into it:
> | +
> | +[cb] <major>:(<minor>|*) [r-][w-]
> | + ^          ^               ^
> | + |          |               |
> | + |          |               +--- access rights (1)
> | + |          |
> | + |          +-- device major and minor numbers (2)
> | + |
> | + +-- device type (character / block)
> | +
> | +1) The access rights set to '--' remove the device from the group's
> | +access list, so that it will not even be shown in this file later.
> | +
> | +2) Setting the minor to '*' grants access to all the minors for
> | +particular major.
> | +
> | +When reading from it, one may see something like
> | +
> | +	c 1:5 rw
> | +	b 8:* r-
> | +
> | +Security issues, concerning who may grant access to what are governed
> | +at the cgroup infrastructure level.
> | +
> | +
> | +Examples:
> | +
> | +1. Grand full access to /dev/null
> 
> Grant.

:)

> | +	# echo c 1:3 rw > /cgroups/<id>/devices.permissions
> | +
> | +2. Grant the read-only access to /dev/sda and partitions
> | +	# echo b 8:* r- > ...
> 
> This grants access to all scsi disks, sda..sdp and not just 'sda' right ?

Well, yes. I'll fix the comment like ;Grant the RO access to scsi disks.

> | +
> | +3. Change the /dev/null access to write-only
> | +	# echo c 1:3 -w > ...
> | +
> | +4. Revoke access to /dev/sda
> | +	# echo b 8:* -- > ...
> | +
> | +
> | +		Written by Pavel Emelyanov <xemul at openvz.org>
> | +
> | diff --git a/fs/Makefile b/fs/Makefile
> | index 7996220..5ad03be 100644
> | --- a/fs/Makefile
> | +++ b/fs/Makefile
> | @@ -64,6 +64,8 @@ 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..48c5f69
> | --- /dev/null
> | +++ b/fs/devscontrol.c
> | @@ -0,0 +1,314 @@
> | +/*
> | + * devscontrol.c - Device Controller
> | + *
> | + * Copyright 2007 OpenVZ SWsoft Inc
> | + * Author: Pavel Emelyanov <xemul at openvz dot 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 {
> | +	/*
> | +	 * The subsys state to build into cgrous infrastructure
> | +	 */
> 
> ... into cgroups
> 
> | +	struct cgroup_subsys_state css;
> | +
> | +	/*
> | +	 * The maps of character and block devices. They provide a
> | +	 * map from dev_t-s to struct cdev/gendisk. See fs/char_dev.c
> | +	 * and block/genhd.c to find out how the ->open() callbacks
> | +	 * work when opening a device.
> | +	 *
> | +	 * Each group will have its onw maps, and at the open()
> 
> own maps
> 
> | +	 * time code will lookup in this map to get the device
> | +	 * and permissions by its dev_t.
> | +	 */
> | +	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);
> | +}
> 
> 'devs' as prefix/suffix does not look very clear.
> 
> How about css_to_devs_cg() ?  Similarly below for dev_cg_create(),
> dev_cg_destroy() ?

These names are internal to devscontrol.c, so I'd like to keep them
as short as possible.

Thanks,
Pavel
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