linux-kernel - Re: [PATCH 0/3] HID: make raw output callback more flexible

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Message-ID: <87bpg7glf7.fsf@troilus.org>
Date:	Tue, 02 Feb 2010 20:25:32 -0500
From:	Michael Poole <mdpoole@...ilus.org>
To:	Bastien Nocera <hadess@...ess.net>,
	Marcel Holtmann <marcel@...tmann.org>
Cc:	Jiri Kosina <jkosina@...e.cz>, "Gunn\, Brian" <bgunn@...ekai.com>,
	Ping <pinglinux@...il.com>, linux-kernel@...r.kernel.org,
	BlueZ development <linux-bluetooth@...r.kernel.org>
Subject: Re: [PATCH 0/3] HID: make raw output callback more flexible

Bastien Nocera writes:

> On Fri, 2010-01-29 at 19:46 -0500, Michael Poole wrote:
>> > [1]: Comments on the patch at
>> > http://thread.gmane.org/gmane.linux.bluez.kernel/4279 would be
>> > appreciated
>> 
>> This patch does not work for me.  Before, the first time after each
>> boot
>> that I tried to connect to an Apple Magic Mouse, it failed with -14
>> (EFAULT).  With this patch, it fails with -22 (EINVAL) instead.  The
>> -EFAULT *was* due to hidp_parse()'s copy_from_user().  I have not
>> looked
>> yet to see where the -EINVAL is coming from -- would that help?  (Both
>> with and without your patch, the second attempt to connect works.) 
>
> I don't get -EFAULT anymore (it was failing to copy the rd_data from
> user-space), but I do get -EINVALs now. I haven't investigated it
> though. My guess is that the hid parser fails.
>
> Could you compare the sizes of the data gathered in user-space?

The bug mysteriously disappeared when I tried to apply kgdb to the
problem.  Your patch won't do the trick because the hidp_connadd_req
structure has also gone out of scope by the time the hid-(whatever)
probe function is called.  The ioctl has returned to the user, but the
new hid-(whatever) module is not yet loaded.

That is, the sequence looks like this:

  - Application triggers hidp_sock_ioctl(socket, HIDPCONNADD, &connadd).
    - This starts the connection, and returns 0...
    - ... but also indicates that some other module needs to be loaded.

  - User-space loads the appropriate module does init_module()...
    - which calls the module_init() function...
    - which calls hid_register_driver()...
    - which eventually attaches the new driver to the device...
    - triggering the module's probe() function to call hid_parse()...
    - hid_parse() fails because its hidp_connadd_req is gone.

Marcel, I think this is a case where the subsystem maintainer should
make the call on how to fix it.  I can write and locally test a patch,
but I don't want to assume that (for example) the desired solution is to
keep a copy of the Report descriptor in the hidp_session.  The USB HID
core sends a GET_DESCRIPTOR request for the Report descriptor, which
isn't practical here because that descriptor is only available through
SDP.

Michael Poole
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