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SubjectRe: [PATCH 2/2] tracefs: Only clobber mode/uid/gid on remount if asked
On Fri, 26 Aug 2022 17:44:17 -0700
Brian Norris <briannorris@chromium.org> wrote:

> Users may have explicitly configured their tracefs permissions; we
> shouldn't overwrite those just because a second mount appeared.
>
> Only clobber if the options were provided at mount time.
>
> Note: the previous behavior was especially surprising in the presence of
> automounted /sys/kernel/debug/tracing/.
>
> # Don't change /sys/kernel/tracing/ permissions on automount.
> umount /sys/kernel/debug/tracing/

BTW, I noticed that the above doesn't do anything. That is,
you cannot unmount tracefs from /sys/kernel/debug/tracing.

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

# umount /sys/kernel/debug/tracing/

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


-- Steve



> stat /sys/kernel/debug/tracing/.
>
> # Don't change /sys/kernel/tracing/ permissions.
> mount -t tracefs none /mnt/foo
>
> # Change /sys/kernel/tracing/ mode and uid, but not gid.
> mount -t tracefs -o uid=bar,mode=0750 none /mnt/baz
>
> Signed-off-by: Brian Norris <briannorris@chromium.org>
> ---
>

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