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Subject: Re: RT scheduling and a way to make a process hang, unkillable
From: Peter Zijlstra <peterz@infradead.org>
To: Corey Hickey <bugfood-ml@fatooh.org>
Cc: linux-kernel@vger.kernel.org, Dhaval Giani <dhaval@linux.vnet.ibm.com>
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References: <4997672B.1000301@fatooh.org>
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Date: Sun, 15 Feb 2009 12:24:56 +0100
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On Sat, 2009-02-14 at 16:51 -0800, Corey Hickey wrote:
> Hello,
> 
> I've encountered a bit of a problem in recent kernels that include
> "Group scheduling for SCHED_RR/FIFO": it is possible for a process run
> by root to hang itself and become unkillable--even by a 'kill -9'.
> 
> The following kernel options must be set:
> CONFIG_GROUP_SCHED=y
> CONFIG_RT_GROUP_SCHED=y
> CONFIG_USER_SCHED=y
> 
> The procedure is for a program to:
> 1. run as root
> 2. set SCHED_FIFO
> 3. change UID to a user with no realtime CPU share allocated

Hmm, setuid() should fail in that situation.

/me goes peek at code.

Can't find any code to make that happen, Dhaval didn't we fix that at
one point?