linux-kernel - Re: [PATCH 2/2] fix sys_unshare()+SEM_UNDO: perform an implicit CLONE_SYSVSEM in CLONE

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Date:	Tue, 15 Apr 2008 20:42:31 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>
Subject: Re: [PATCH 2/2] fix sys_unshare()+SEM_UNDO: perform an implicit CLONE_SYSVSEM
 in CLONE_NEWIPC

Serge E. Hallyn wrote:
>
>> Thus all apps right now call unshare(CLONE_NEWIPC|&~CLONE_SYSVSEM).
>> This combination doesn't make much sense. Even worse - it easily causes a 
>> kernel oops.
>> Thus my fix is twofold:
>> - add support for unshare(CLONE_SYSVSEM).
>> - implicitely add CLONE_SYSVSEM to all calls that set CLONE_NEWIPC.
>>     
>
> Right but your fix ignores the fact that you can achieve the same
> result as unshare(CLONE_NEWIPC&~CLONE_SYSVSEM) by doing
> clone(CLONE_NEWIPC|CLONE_SYSVSEM).
>   
Duh, I overlooked that part. You are right.
Hmm - what's the best fix? Implicitely add CLONE_SYSVSEM or return 
-EINVAL if CLONE_SYSVSEM is not set?
I don't care.
Could you write a patch? I probably won't have enough time until the 
next weekend.

>> I have decided against that, it breaks the current ABI.
>> And we gain virtually nothing - most if not all unshare users will be 
>> single threaded apps that do not use sysvsem at all, and even most sysvsem 
>> users do not use SEM_UNDO.
>>     
>
> And most importantly sharing your semundo list but not your sems with
> your parent is silly!
It's only 99% silly: a task might want to have not-yet undone changes in its parent's namespace.
As soon as the task exit, they are undone. Probably the most complex way to implement wait4().


--
Manfred
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