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Date: Fri, 17 Mar 2006 20:44:54 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
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To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@osdl.org>, linux-kernel@vger.kernel.org,
       Andrew Morton <akpm@osdl.org>, Janak Desai <janak@us.ibm.com>,
       Al Viro <viro@ftp.linux.org.uk>, Christoph Hellwig <hch@lst.de>,
       Michael Kerrisk <mtk-manpages@gmx.net>, Andi Kleen <ak@muc.de>,
       Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH] unshare: Use rcu_assign_pointer when setting sighand
References: <m1y7za9vy3.fsf@ebiederm.dsl.xmission.com> <m1pskm9tz9.fsf@ebiederm.dsl.xmission.com>
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"Eric W. Biederman" wrote:
> 
> @@ -1573,7 +1573,7 @@ asmlinkage long sys_unshare(unsigned lon
> 
>                 if (new_sigh) {
>                         sigh = current->sighand;
> -                       current->sighand = new_sigh;
> +                       rcu_assign_pointer(current->sighand, new_sigh);
>                         new_sigh = sigh;
>                 }

Isn't it better to just replace this code with
'BUG_ON(new_sigh != NULL)' ?

It is never executed, but totally broken, afaics.
task_lock() has nothing to do with ->sighand changing.

Oleg.