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

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Message-ID: <4B5EFA7A.9080800@us.ibm.com>
Date:	Tue, 26 Jan 2010 06:21:46 -0800
From:	Darren Hart <dvhltc@...ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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

KOSAKI Motohiro wrote:
>> 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.
> 
> May I confirm your mention?
> 
> If we can accept EFAULT, we don't need any change. my previous futex patch
> already did. because 1) VDSO is alwasys read-only mapped 2) write mode
> get_user_pages_fast() against read-only pte/vma return EFAULT.
> 
> Current linus and stable tree don't cause Mark's original problem. instead, just
> return EFAULT. (Well, I'm sorry. my previous mail was unclear. I wrote v2.6.31 test
> result)
> 
> If you can't accept EFAULT, we need to add vdso specific logic into get_futex_key().
> Is this your intention?

That was my intention, but after looking at the glibc source, I don't 
see any reason for EINVAL over EFAULT. I apparently mis-remembered 
something there. EFAULT is fine.

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