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On 02/17/2011 10:27 PM, Olivier Guilyardi wrote:

quoted text
> On 02/17/2011 09:48 PM, Paul Davis wrote:

>> On Thu, Feb 17, 2011 at 3:40 PM, Olivier Guilyardi  wrote:

>>

>> in earlier versions of 2.6, the kernel patch to allow SCHED_FIFO for

>> everyone was incredibly simple. i recall kjetil posting a couple of

>> lines, at most.

>>

>> whether this a security risk depends on which other parts of the

>> kernel android uses. on regular linux, its no longer possible for any

>> process to steal all the CPU time. there are files in the /proc/sys

>> filesystem that control the amount available.

>>

>> note that patching the kernel in this way means that any process by

>> any user can get SCHED_FIFO so its hardly clear that this is actually

>> any better than using rlimits from a security perspective.

>

> I know that patching is not a requirement anymore for realtime on Linux. But I

> was just asking about some possible security enhancement patch.

>

> What are those files in /proc/sys that you mention?

Hi Olivier,
Paul must have been talking about
/proc/sys/kernel/sched_rt_runtime_us

/proc/sys/kernel/sched_rt_period_us
/Documentation/scheduler/sched-rt-group.txt
> Also I'm a bit confused about rlimits. IIUC, realtime processes always run

quoted text
> before non-realtime processes. And the realtime priority as set with

> set_rlimits() only affects the priority of realtime processes between each

> other. It doesn't seem to affect the priority between realtime and non-realtime

> processes.

>

> I've been told than on OSX, when a process runs in realtime, it is only allowed

> to use a certain ratio of CPU time. And if it goes over this limit, it loses its

> priority. Is there something comparable on Linux? Maybe with the files in

> /proc/sys that you mention?

not exactly. /proc kernel.sched_rt_runtime_us globally limits realtime

scheduling time. To limit it per process you'll need cgroups.
> What I'm looking for is some way to say: ok, this thread can run in realtime,

quoted text
> but if it ever uses more than 10% of CPU time it will lose realtime priority.

>

> Aside of that, what about locks? I've many times been told that one mustn't do

> anything that could block in a realtime thread. What are the consequences of

> that? Could a malicious app freeze the system by blocking in a realtime thread?

>

> --

>   Olivier

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