linux-kernel - Re: [PATCH] print_worker_info: Handle pointer with more care

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Message-ID: <520E58F3.4080309@nod.at>
Date:	Fri, 16 Aug 2013 18:53:07 +0200
From:	Richard Weinberger <richard@....at>
To:	Tejun Heo <tj@...nel.org>
CC:	linux-kernel@...r.kernel.org, Jeff Dike <jdike@...toit.com>,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH] print_worker_info: Handle pointer with more care

Am 16.08.2013 18:45, schrieb Tejun Heo:
> Hello,
> 
> On Fri, Aug 16, 2013 at 06:38:58PM +0200, Richard Weinberger wrote:
>>> cc'ing uml people.  Hey, guys, workqueue uses proble_kernel_read() to
>>> print out workqueue related information during oops because those
>>> events are completely asynchronous and workqueue states may not be
>>> consistently accessible.  It seems like uml doesn't implement
>>> probe_kernel_read() and tries direct derference of incorrect pointers
>>> leading to its own oops.  Maybe uml should check whether the memory is
>>> mapped from probe_kernel_read()?
>>
>> You are already talking to UML people. ;)
> 
> Ooh... :)
> 
>> Anyway, I'll investigate into that.
>> What I see so far is that pwq is NULL after probe_kernel_read().
> 
> Yeah, and that should be fine.  &pwq->wq would be just an offset of wq

Yep. Now my brain also parsed the C notation correctly.
Sorry for the completely wrong patch.

> from NULL which is an invalid pointer but probe_kernel_read() should
> be able to handle that and probably just return 0 or -1 (all bits
> set).  I *think* what's necessary is making probe_kernel_read() use
> mincore() to fine out whether the requested address is mapped (it
> should return -EFAULT if not) and try to dereference the address iff
> it's mapped.

UML needs a custom probe_kernel_read()? Fine. :)

Thanks,
//richard

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