linux-kernel - Re: [PATCH 2/2] tracefs: Only clobber mode/uid/gid on remount if asked

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Date:   Wed, 7 Sep 2022 18:45:24 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Brian Norris <briannorris@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] tracefs: Only clobber mode/uid/gid on remount if
 asked

On Wed, 7 Sep 2022 14:53:42 -0700
Brian Norris <briannorris@...omium.org> wrote:

> Try these:
> 
>   umount /sys/kernel/debug/tracing/
>   grep tracefs /proc/mounts
>   stat /sys/kernel/debug/tracing/.
>   grep tracefs /proc/mounts
> 
> The first and the second grep will give you different results.

How about this:

  # grep tracefs /proc/mounts
tracefs /sys/kernel/tracing tracefs rw,nosuid,nodev,noexec,relatime 0 0
tracefs /sys/kernel/debug/tracing tracefs rw,nosuid,nodev,noexec,relatime 0 0

 # umount /sys/kernel/debug/tracing 
 # grep tracefs /proc/mounts
tracefs /sys/kernel/tracing tracefs rw,nosuid,nodev,noexec,relatime 0 0

 # ls /sys/kernel/debug/tracing
available_events            hwlat_detector       set_event_notrace_pid   trace_clock
available_filter_functions  instances            set_event_pid           trace_marker
available_tracers           kprobe_events        set_ftrace_filter       trace_marker_raw
buffer_percent              kprobe_profile       set_ftrace_notrace      trace_options
buffer_size_kb              max_graph_depth      set_ftrace_notrace_pid  trace_pipe
buffer_total_size_kb        options              set_ftrace_pid          trace_stat
current_tracer              osnoise              set_graph_function      tracing_cpumask
dynamic_events              per_cpu              set_graph_notrace       tracing_max_latency
dyn_ftrace_total_info       printk_formats       snapshot                tracing_on
enabled_functions           README               stack_max_size          tracing_thresh
error_log                   recursed_functions   stack_trace             uprobe_events
eval_map                    saved_cmdlines       stack_trace_filter      uprobe_profile
events                      saved_cmdlines_size  synthetic_events        user_events_data
free_buffer                 saved_tgids          timestamp_mode          user_events_status
function_profile_enabled    set_event            trace

 # grep tracefs /proc/mounts
tracefs /sys/kernel/tracing tracefs rw,nosuid,nodev,noexec,relatime 0 0
tracefs /sys/kernel/debug/tracing tracefs rw,nosuid,nodev,noexec,relatime 0 0

So it appears that accessing debugfs/tracing will automatically remount it.

Lean something new everyday! ;-)

-- Steve