linux-kernel - Re: debugfs vs. device removal

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Message-ID: <alpine.LSU.2.20.1701191700150.25515@cbobk.fhfr.pm>
Date:   Thu, 19 Jan 2017 17:03:48 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
cc:     Omar Sandoval <osandov@...ndov.com>, linux-kernel@...r.kernel.org
Subject: Re: debugfs vs. device removal

On Thu, 19 Jan 2017, Greg Kroah-Hartman wrote:

> > In the block layer, we abuse sysfs to export some per-device debugging
> > information. I was looking into moving this to debugfs, but I realized
> > that debugfs doesn't have a mechanism to ensure that a file associated
> > with a device is safe to use when the device is removed. 
> 
> What do you mean by "safe"?  The race conditions where you remove a file
> and still have it open should all now be resolved in 4.8 and 4.9, di dwe
> miss something?

This is something else -- Omar is right, hid-debugfs interface is buggy. 
It basically doesn't synchronize the data dumping with device removal, so 
if device is removed and deallocated and the race is hit, it tries to 
dereference struct hid_device which has already been freed.

I'll look into fixing this later today or tomorrow. Basically we'd need to 
synchronize between hid_remove_device() and anything in hid-debug and 
whenever removal is pending, not to try to get any data out of it any more 
and bail immediately. Something like rwlock (debugfs being the reader and 
device removal being the writer) should work.

Thanks,

-- 
Jiri Kosina
SUSE Labs