Re: /sys/kernel/tracing/events permissions

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Mon, Nov 22, 2021 at 11:55:11AM +0100, Milian Wolff escreveu:
> Hello,
> 
> a recent kernel update on my arch system has lead to breakage with regard to 
> accessing trace events from perf. For example:

I saw some recent changes but haven't yet tried to reflect it into the
perf tools:

commit 79ef0c00142519bc34e1341447f3797436cc48bf
Merge: d54f486035fd89f1 feea69ec121f0670
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon Nov 1 20:05:19 2021 -0700

    Merge tag 'trace-v5.16' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

    Pull tracing updates from Steven Rostedt:
<SNIP>
     - Have tracefs allow owner and group permissions by default (only
       denying others). There's been pressure to allow non root to tracefs
       in a controlled fashion, and using groups is probably the safest.
<SNIP>

 
> ```
> $ 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?

Rostedt, I looked for documentation for this on Documentation/trace/ but
couldn't find, so its just a matter of creating a 'tracing' group and
adding users to this group that then can use tracefs files?

- Arnaldo