Re: [PATCH v6 2/4] tracing: Make the backup instance non-reusable

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 11 Feb 2026 10:42:44 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue, 10 Feb 2026 14:14:15 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > Hmm, OK. Now I found how sysfs handles it.
> > 
> > 	/*
> > 	 * For regular files, if the opener has CAP_DAC_OVERRIDE, open(2)
> > 	 * succeeds regardless of the RW permissions.  sysfs had an extra
> > 	 * layer of enforcement where open(2) fails with -EACCES regardless
> > 	 * of CAP_DAC_OVERRIDE if the permission doesn't have the
> > 	 * respective read or write access at all (none of S_IRUGO or
> > 	 * S_IWUGO) or the respective operation isn't implemented.  The
> > 	 * following flag enables that behavior.
> > 	 */
> > 	KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK	= 0x0002,
> > 
> > So for the similar reason, I will make tracefs to check the permission
> > even if CAP_DAC_OVERRIDE is set. (But this check should be done in general,
> > instead of each open() operation)
> > 
> 
> I don't believe this is the same. This is about an instance being truly
> read only. The instance is special, not the files. Note, permissions can
> be changed by root too.

Ah, OK. Let me add read only checks in all related .open operations.

> After applying your patches, I did the following:
> 
> ~# cd /sys/kernel/tracing/instances/backup/
> ~# ls -l current_tracer
> -r--r----- 1 root root 0 Feb 11 10:29 current_tracer
> 
> ~# cat current_tracer 
> nop
> 
> ~# cat trace
> # tracer: nop
> #
> # entries-in-buffer/entries-written: 0/0   #P:8
> #
> #                                _-----=> irqs-off/BH-disabled
> #                               / _----=> need-resched
> #                              | / _---=> hardirq/softirq
> #                              || / _--=> preempt-depth
> #                              ||| / _-=> migrate-disable
> #                              |||| /     delay
> #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
> #              | |         |   |||||     |         |
> 
> ~# chmod 664 current_tracer
> ~# ls -l current_tracer
> -rw-rw-r-- 1 root root 0 Feb 11 10:29 current_tracer

Ah, OK...

> 
> ~# echo function > current_tracer
> ~# cat current_tracer 
> function
> 
> ~# cat trace
> # tracer: function
> #
> # entries-in-buffer/entries-written: 1750306/2076556   #P:8
> #
> #                                _-----=> irqs-off/BH-disabled
> #                               / _----=> need-resched
> #                              | / _---=> hardirq/softirq
> #                              || / _--=> preempt-depth
> #                              ||| / _-=> migrate-disable
> #                              |||| /     delay
> #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
> #              | |         |   |||||     |         |
>             bash-1056    [001] .....   231.448852: mutex_unlock <-tracing_set_tracer
>           <idle>-0       [002] ...1.   231.448853: arch_cpu_idle_exit <-do_idle
> ##### CPU 7 buffer started ####
>           <idle>-0       [007] ...1.   231.448853: arch_cpu_idle_exit <-do_idle
>             bash-1056    [001] .....   231.448854: __mutex_unlock_slowpath <-tracing_set_tracer
>           <idle>-0       [002] d..1.   231.448855: arch_cpu_idle_enter <-do_idle
>           <idle>-0       [007] d..1.   231.448855: arch_cpu_idle_enter <-do_idle
>           <idle>-0       [007] d..1.   231.448855: tsc_verify_tsc_adjust <-arch_cpu_idle_enter
>           <idle>-0       [002] d..1.   231.448855: tsc_verify_tsc_adjust <-arch_cpu_idle_enter
>             bash-1056    [001] d....   231.448856: fpregs_assert_state_consistent <-arch_exit_to_user_mode_prepare
>           <idle>-0       [007] d..1.   231.448856: local_touch_nmi <-do_idle
>           <idle>-0       [002] d..1.   231.448856: local_touch_nmi <-do_idle
>             bash-1056    [001] d....   231.448856: switch_fpu_return <-arch_exit_to_user_mode_prepare
>           <idle>-0       [007] d..1.   231.448856: rcu_nocb_flush_deferred_wakeup <-do_idle
>           <idle>-0       [002] d..1.   231.448856: rcu_nocb_flush_deferred_wakeup <-do_idle
>           <idle>-0       [007] d..1.   231.448857: cpuidle_get_cpu_driver <-do_idle
>           <idle>-0       [002] d..1.   231.448857: cpuidle_get_cpu_driver <-do_idle
>           <idle>-0       [007] d..1.   231.448857: cpuidle_not_available <-do_idle
> [..]
> 
> Not too read only!
> 
> I change permissions all the time for tracefs files, so I don't want
> that changed.
> 
> This is not the same as sysfs. Let's keep it simple. Have all the open
> callers that can do writes return error -EACCES if a file allows writes
> and is open for write, but is part of the read only instance.

OK.

Thank you,
> 
> -- Steve
>  


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>