linux-kernel - Re: debugfs vs. device removal

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Message-ID: <20170119155345.GA11660@kroah.com>
Date:   Thu, 19 Jan 2017 16:53:45 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Omar Sandoval <osandov@...ndov.com>
Cc:     linux-kernel@...r.kernel.org, Jiri Kosina <jikos@...nel.org>
Subject: Re: debugfs vs. device removal

On Thu, Jan 19, 2017 at 07:48:41AM -0800, Omar Sandoval wrote:
> Hi,
> 
> 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?

> At a quick glance, HID has some per-device information in debugfs.
> However, I don't see any sort of protection against a device being
> removed. I was easily able to trigger an oops by reading from
> /sys/kernel/debug/hid/*/rdesc in a loop and removing the USB device
> (trace attached).
> 
> How can I safely export per-device debugging information to debugfs?

Try 4.9 and see if you can still reproduce this, it should be fixed.  If
not, we might have forgotten to include the .owner field for the hid
debugfs file...

thanks,

greg k-h