On Tue, 19 Aug 2025 10:51:52 +0000
Luo Gengkun [off-list ref] wrote:

Both tracing_mark_write and tracing_mark_raw_write call
__copy_from_user_inatomic during preempt_disable. But in some case,
__copy_from_user_inatomic may trigger page fault, and will call schedule()
subtly. And if a task is migrated to other cpu, the following warning will

Wait! What?

__copy_from_user_inatomic() is allowed to be called from in atomic context.
Hence the name it has. How the hell can it sleep? If it does, it's totally
broken!

Now, I'm not against using nofault() as it is better named, but I want to
know why you are suggesting this change. Did you actually trigger a bug here?

be trigger:
        if (RB_WARN_ON(cpu_buffer,
                       !local_read(&cpu_buffer->committing)))

An example can illustrate this issue:

process flow						CPU
---------------------------------------------------------------------

tracing_mark_raw_write():				cpu:0
   ...
   ring_buffer_lock_reserve():				cpu:0
      ...
      cpu = raw_smp_processor_id()			cpu:0
      cpu_buffer = buffer->buffers[cpu]			cpu:0
      ...
   ...
   __copy_from_user_inatomic():				cpu:0
      ...
      # page fault
      do_mem_abort():					cpu:0

Sounds to me that arm64 __copy_from_user_inatomic() may be broken.

         ...
         # Call schedule
         schedule()					cpu:0
	 ...
   # the task schedule to cpu1
   __buffer_unlock_commit():				cpu:1
      ...
      ring_buffer_unlock_commit():			cpu:1
	 ...
	 cpu = raw_smp_processor_id()			cpu:1
	 cpu_buffer = buffer->buffers[cpu]		cpu:1

As shown above, the process will acquire cpuid twice and the return values
are not the same.

To fix this problem using copy_from_user_nofault instead of
__copy_from_user_inatomic, as the former performs 'access_ok' before
copying.

Fixes: 656c7f0d2d2b ("tracing: Replace kmap with copy_from_user() in trace_marker writing")

The above commit was intorduced in 2016. copy_from_user_nofault() was
introduced in 2020. I don't think this would be the fix for that kernel.

So no, I'm not taking this patch. If you see __copy_from_user_inatomic()
sleeping, it's users are not the issue. That function is.

-- Steve

Signed-off-by: Luo Gengkun <redacted>