Passing references to kobjects between userland and kernel

Passing references to kobjects between userland and kernel

[Daniele orlandi]

Hello,

I'm trying to figure out what is the correct way to pass references to 
kobjects between userland and kernel space.

I have my big object hierarchy of kobjects representing a TDM interconnect 
graph with channels, crossconnectors, physical ports and so on.
The main objects are nodes and archs; archs connect two nodes together.

The hierarchy is exported to sysfs.

>From userland I want to tell the kernel "Connect node X to node Y".

The first problem is deciding what interface to use; currently I'm using an 
ioctl() but I'm open to suggestions.

The other issue is how to identify objects X and Y in the ioctl() request, 
here are the possible ways I've examined:

1- Path
-------

Every kobject registered in sysfs has its own path. I can use path_lookup() 
and lookup the object by its kobj.dentry.

Pros: In userland it's easy to identify kobjects by their path in sysfs. The 
path is unambiguous and already available.

Cons: Passing the pathname to the kernel requires a big ioctl() body 
(theoretically at least 2*MAX_PATH using a fixed-length structure).

Doing the reverse mapping (from kernel to userspace) is not easy since you 
need to know from which vfsmount to generate the path with d_path().

2- Relative path
----------------

Pros: No need to consider where sysfs is mounted and would also work if the 
kobjects are not registeres in sysfs.

Cons: There should be specific functions to parse/generate a pathname that is 
specific to kobject hierarchy.

3- Unique ID
------------

A unique ID, probably a integer value can be assigned to each object.

Pros: much more compact, do not need parsing

Cons: Needs an 'id' attribute in sysfs for every object and a reverse mapping 
(done with symlinks?) to allow an userland application to map IDs to paths 
and vice-versa. Unique ID allocation needs an allocator (list, bitmap, etc) 
either global or for every namespace.

4- kobject's pointer
------------------
The kobject's pointer could opaquely be seen as a unique identifier.

Pros: Compact, no need for storage.

Cons: Probably insane, exposes kernel internals to the userland, a developer 
may be tempted to deference the user-provided value directly :)

So, are there any other ways? If not, which one would be advisable among 
these?

Bye,

-- 
  Daniele "Vihai" Orlandi

Re: Passing references to kobjects between userland and kernel

[Greg KH]

On Wed, Jun 14, 2006 at 04:26:38PM +0200, Daniele orlandi wrote:
> 
> Hello,
> 
> I'm trying to figure out what is the correct way to pass references to 
> kobjects between userland and kernel space.

Use the kobject_uevent() call from kernelspace to let userspace know
whatever you want it to.  That is what it is there for :)

> 
> I have my big object hierarchy of kobjects representing a TDM interconnect 
> graph with channels, crossconnectors, physical ports and so on.
> The main objects are nodes and archs; archs connect two nodes together.
> 
> The hierarchy is exported to sysfs.
> 
> From userland I want to tell the kernel "Connect node X to node Y".

Then use the names of the kobjects within your subdirectory.  They have
to be unique so you should be safe.

Or use configfs :)

thanks,

greg k-h

Re: Passing references to kobjects between userland and kernel

[Daniele Orlandi]

On Saturday 17 June 2006 01:58, Greg KH wrote:
>
> Use the kobject_uevent() call from kernelspace to let userspace know
> whatever you want it to.  That is what it is there for :)

kobject_uevent() is fine if I want to asynchronously notify the user space of 
an event.

What I need is a synchronous bidirectional interface, e.g. I tell the kernel 
"connect node X with node Y" and I get back the resulting pipeline 
identifier.

> Or use configfs :)

Configfs is very interesting, it's surely well suited for several tasks I'm 
doing and I will add support for it into my subsystem.

What it's missing is the ability to create non-persistent configurations, 
bound to a process and disappearing along with the process that created them.

For example, my process creates pipelines between nodes using a ioctl() 
interface. If the process dies unexpectedly the file descriptor is 
automatically closed and I can release all the pipelines. I don't see a way I 
could accomplish the same with configfs or sysfs. 

OTOH, for what concerns persistent pipelines, configfs is quite good. It would 
be even better if I could create links from configfs objects to sysfs 
objects.

Bye,

-- 
  Daniele "Vihai" Orlandi

Re: Passing references to kobjects between userland and kernel

[Greg KH]

On Tue, Jun 20, 2006 at 01:48:54AM +0200, Daniele Orlandi wrote:
> On Saturday 17 June 2006 01:58, Greg KH wrote:
> >
> > Use the kobject_uevent() call from kernelspace to let userspace know
> > whatever you want it to.  That is what it is there for :)
> 
> kobject_uevent() is fine if I want to asynchronously notify the user space of 
> an event.
> 
> What I need is a synchronous bidirectional interface, e.g. I tell the kernel 
> "connect node X with node Y" and I get back the resulting pipeline 
> identifier.

Why do you feel that this is a requirement?  What exactly are you trying
to do?

thanks,

greg k-h

Re: Passing references to kobjects between userland and kernel

[Daniele Orlandi]

On Tuesday 20 June 2006 01:53, Greg KH wrote:
>
> Why do you feel that this is a requirement?  What exactly are you trying
> to do?

I have my channels interconnect graph, nicely exported to sysfs, and I just 
need an interface that an application could use to request connections 
between two nodes of this graph.

The result of the connection is a pipeline object, also exported to sysfs, 
that the application will use to do further work on the connection.

Currently I'm using an ioctl() but, given that ioctls seem to be a no-no for 
future projects, given that there is great effort in having new and better 
interfaces, I'm trying to figure out what is the best way to do what I need.

thanks,
Bye,

-- 
  Daniele Orlandi