[Devel] Re: [PATCH] Mark timer_stats as incompatible with multiple pid namespaces

[Eric W. Biederman]

Ingo Molnar <mingo at elte.hu> writes:

> * Eric W. Biederman <ebiederm at xmission.com> wrote:
>
>> /proc/timer_stats currently reports the user of a timer by pid, which 
>> is a reasonable approach.  However if you are not in the initial pid 
>> namespace the pid that is reported is nonsense.
>> 
>> Therefore until we can make timer_stats pid namespace safe just 
>> disable it in the build if pid namespace support is selected so we at 
>> least know we have a conflict.
>
> What the heck??? Please solve this properly instead of hiding it. 
> /proc/timer_stats is damn useful and it's a must-have for powertop to 
> work.

Hmm.  Perhaps the dependency conflict should go in the other direction
then.

My goal is to document the issue while a proper fix is being written.
I have known about this for all of about 1 day now.  It was added since
last time I went through the kernel and made a thorough sweep of pid users.

What the proper fix is isn't even obvious at this point.
Possibly it is making /proc/timer_stats disappear in child pid namespaces.
  Which we don't currently have the infrastructure fore.
Possibly it is reworking the stats collection so we store a struct pid *
instead of a pid_t value.  So we would know if the reader of the value
can even see processes you have collected stats for.

It is going to take a bit to digest what is going on and solve this properly.

In the same vein do we actively have interesting user space programs
using /proc/sched_debug?  It is the same class of problem.  Yet
another interface talking to user space with pids.

Eric
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