CODEWITHSUNDEEP
linux-kernelx86profilingperfintel-pmu
BeeOnRope
BeeOnRope

Reputation: 64895

What restriction is perf_event_paranoid == 1 actually putting on x86 perf?

Newer Linux kernels have a sysfs tunable /proc/sys/kernel/perf_event_paranoid which allows the user to adjust the available functionality of perf_events for non-root users, with higher numbers being more secure (offering correspondingly less functionality):

From the kernel documenation we have the following behavior for the various values:

perf_event_paranoid:

Controls use of the performance events system by unprivileged users (without CAP_SYS_ADMIN). The default value is 2.

-1: Allow use of (almost) all events by all users Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK

>=0: Disallow ftrace function tracepoint by users without CAP_SYS_ADMIN Disallow raw tracepoint access by users without CAP_SYS_ADMIN

>=1: Disallow CPU event access by users without CAP_SYS_ADMIN

>=2: Disallow kernel profiling by users without CAP_SYS_ADMIN

I have 1 in my perf_event_paranoid file which should "Disallow CPU event access" - but what does that mean exactly?

A plain reading would imply no access to CPU performance counter events (such as Intel PMU events), but it seems I can access those just fine. For example:

$ perf stat sleep 1

 Performance counter stats for 'sleep 1':

          0.408734      task-clock (msec)         #    0.000 CPUs utilized          
                 1      context-switches          #    0.002 M/sec                  
                 0      cpu-migrations            #    0.000 K/sec                  
                57      page-faults               #    0.139 M/sec                  
         1,050,362      cycles                    #    2.570 GHz                    
           769,135      instructions              #    0.73  insn per cycle         
           152,661      branches                  #  373.497 M/sec                  
             6,942      branch-misses             #    4.55% of all branches        

       1.000830821 seconds time elapsed

Here, many of the events are CPU PMU events (cycles, instructions, branches, branch-misses, cache-misses).

If these aren't the CPU events being referred to, what are they?

Upvotes: 35

Views: 44275

Answers (1)

Zulan
Zulan

Reputation: 22660

In this case CPU event refers to monitoring events per CPU rather than per task. For perf tools this restricts the usage of

-C, --cpu=
    Count only on the list of CPUs provided. Multiple CPUs can be provided as a comma-separated list with no space: 0,1.
    Ranges of CPUs are specified with -: 0-2. In per-thread mode, this option is ignored. The -a option is still necessary
    to activate system-wide monitoring. Default is to count on all CPUs.

-a, --all-cpus
    system-wide collection from all CPUs (default if no target is specified)

For perf_event_open this considers the following case:

pid == -1 and cpu >= 0
       This measures all processes/threads on the specified CPU.  This requires CAP_SYS_ADMIN capability or a /proc/sys/ker‐
       nel/perf_event_paranoid value of less than 1.

This may be version specific, the cited documentation is from 4.17. This is another related question.

Upvotes: 15

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