linux-kernel - Re: futex() on vdso makes process unkillable

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Date:	Mon, 25 Jan 2010 09:37:35 -0800
From:	Darren Hart <dvhltc@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mark Seaborn <mrs@...hic-beasts.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>
Subject: Re: futex() on vdso makes process unkillable

Peter Zijlstra wrote:
> On Mon, 2010-01-25 at 16:27 +0900, KOSAKI Motohiro wrote:
<snip>
>> Futex should work both file anon anon. however I personally think 
>> vdso is not file nor anon. it is special mappings. nobody defined
>> futex spec on special mappings. (yes, undefined).
>>
>> Personally, I think EINVAL or EFAULT are best result of vdso futexing, like as
>> futexing againt kernel address. but I guess another person have another thinking.
>>
>> I'd like to hear futex folks's opinion.
> 
> Well, my opinion is we should remove the vdso, its ugly as hell :-)
> 
> But I think it would make most sense to extend its definition in the
> direction of it being a file (for all intents and purposes its a special
> DSO -- which unfortunately isn't present in any filesystem).
> 
> [ For all intents and purposes processes can already communicate through
> futexes on the libc space, so being able to do so through the vsdo
> really doesn't add anything ]
> 
> So the problem is that the VDSO pages do not have a page->mapping
> because they lack the actual filesystem part of files, so even if (with
> the recent zero-page patch from Kosaki-san) you make private COWs of the
> VDSO, you'll get stuck in that loop.
> 
> So the prettiest solution is to simply place the vdso in an actual
> filesystem and slowly migrate towards letting userspace map it as a
> regular DSO -- /sys/lib{32,64}/libkernel.so like.
> 
> [ that has the bonus of getting rid of install_special_mapping() ]
> 
> The ugly solution is special casing the vdso in get_futex_key().

I like the creating-a-real-file solution. However, for now (and for 
stable), I think Kosaki's suggestion of EINVAL or EFAULT is a good 
stop-gap. EINVAL might play the best with existing glibc implementations.

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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