/sys/kernel/tracing/events permissions

[Milian Wolff <milian.wolff@kdab.com>]

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Hello,

a recent kernel update on my arch system has lead to breakage with regard to 
accessing trace events from perf. For example:

```
$ perf record -e sched:sched_switch ls 
event syntax error: 'sched:sched_switch'
                     \___ can't access trace events

Error:  No permissions to read /sys/kernel/tracing/events/sched/sched_switch
Hint:   Try 'sudo mount -o remount,mode=755 /sys/kernel/tracing/'

Run 'perf list' for a list of valid events

 Usage: perf record [<options>] [<command>]
    or: perf record [<options>] -- <command> [<options>]

    -e, --event <event>   event selector. use 'perf list' to list available 
events
```

When I then try to run the remount hint from the message above, which used to 
work just fine, then the error persists, because:

```
$ mount | grep tracing
tracefs on /sys/kernel/tracing type tracefs 
(rw,nosuid,nodev,noexec,relatime,mode=755)
tracefs on /sys/kernel/debug/tracing type tracefs 
(rw,nosuid,nodev,noexec,relatime,mode=755)
```

and:

```
$ ls -ld /sys/kernel/tracing/
drwxr-xr-x root root 0 B Mon Nov 22 08:39:15 2021 /sys/kernel/tracing/
$ ls -ld /sys/kernel/tracing/events/
drwxr-x--- root root 0 B Mon Nov 22 08:39:26 2021 /sys/kernel/tracing/events/
```

How should this be handled? Is it now required to manually `chmod` the 
`events` folder, or is there another means to allow non-root to record kernel 
trace events?

Thanks

-- 
Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts

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