Re: [Xen-users] Got permission denied error when calling xenbus_printf function in the frontend driver

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Sun, 2015-08-02 at 17:12 -0400, Jintack Lim wrote:
> Hi,
> 
> I was writing a simple backend/frontend driver,
> and when I ran it, I got this error in dmesg.
> "failed to write error node for device/vsimple/0 (13 writing ring-ref)"

Was this from the front or backend? I think it is the frontend.

> The way I call xenbus_printf is pretty much the same as other drivers.
> I called xenbus_transaction_start before this line.
> err = xenbus_printf(xbt, dev->nodename,
>                                "ring-ref", "%u", info->ring_ref);
> 
> I'm working on Xen 4.5.0, ARM64 architecture.
> I referred this page, and used the same script at the bottom.
> http://wiki.xenproject.org/wiki/Introduction_to_Xen_3.x#Adding_new_device
> _and_triggering_the_probe.28.29_functions
> What could go wrong with permissions?
> 
> This is how my device (vsimple) looks like in Xenstore
> after I ran the script above.
> 
> # xenstore-ls -p /local/domain/7/device
> ...
> vsimple = "" . . . . . . . . . . . . . . . . . . . . . . . .  (n0,r7)
>  0 = ""  . . . . . . . . . . . . . . . . . . . . . . . . . .  (n0,r7)
>   backend-id = "0" . . . . . . . . . . . . . . . . . . . . .  (n0,r7)
>   backend = "/local/domain/0/backend/vsimple/7/0"  . . . . .  (n0,r7)
>   state = "1"  . . . . . . . . . . . . . . . . . . . . . . .  (n0,r7)
> 
> The owner of vsimple is dom0, and domU only has the read permission.
> It's somewhat obvious that I got the permission error..
> The question is how can I fix this?
> 
> I also got errors related to writing new state, and domU is only allowed 
> to read it as shown above again.

The frontend domain needs to be given write permission to the frontend
device directory (or at least some subset, compare your perms against e.g.
a disk directory). Normally the toolstack would arrange for the correct
permissions as part of the creation of both the front and backend
directories.

What is creating /local/domain/<domid>/device/vsimple/<devid> and
/local/domain/<be>/backend/vsimple/<domid>/<devid>?

I should also ask -- what is "vsimple" going to do? It may be that we have
existing things which meet your needs, either libvchan or channels[0] might
be a good fit?

Ian.

[0] http://xenbits.xen.org/docs/unstable/misc/channel.txt

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