Re: [PATCH 2/2] proc: Add /proc/<pid>/timerslack_ns interface

[John Stultz]

On Wed, Feb 17, 2016 at 12:18 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 17 Feb 2016 12:09:08 -0800 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> On Wed, Feb 17, 2016 at 11:35 AM, Andrew Morton
>> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> > On Tue, 16 Feb 2016 17:06:31 -0800 John Stultz <john.stultz@xxxxxxxxxx> wrote:
>> >
>> >> This patch provides a proc/PID/timerslack_ns interface which
>> >> exposes a task's timerslack value in nanoseconds and allows it
>> >> to be changed.
>> >>
>> >> This allows power/performance management software to set timer
>> >> slack for other threads according to its policy for the thread
>> >> (such as when the thread is designated foreground vs. background
>> >> activity)
>> >>
>> >> If the value written is non-zero, slack is set to that value.
>> >> Otherwise sets it to the default for the thread.
>> >>
>> >> This interface checks that the calling task has permissions to
>> >> to use PTRACE_MODE_ATTACH_FSCREDS on the target task, so that we
>> >> can ensure arbitrary apps do not change the timer slack for other
>> >> apps.
>> >
>> > hm.  What the heck is PTRACE_MODE_ATTACH_FSCREDS and why was it chosen?
>>
>> This says the writer needs to have ptrace "attach" level of access,
>> and that it should be checked with fscreds, as is the standard for
>> most /proc things like that.
>
> The only place where PTRACE_MODE_ATTACH_FSCREDS is used in all of Linux
> is /prc/pid/stack.  Makes me curious!

Other uses may be using a combination PTRACE_MODE_ATTACH|PTRACE_MODE_FSCREDS.

>> > The procfs file's permissions are 0644, yes?  So a process's
>> > timer_slack is world-readable?  hm.
>>
>> This should be 600, IMO.
>
> Sounds safer.

Ok. Reworking the patch to use that as well.


Andrew: I saw you added these to -mm already. Would you prefer a fixup
patch ontop, or should I just send out a folded down v3 of the
patchset?

thanks
-john