[RFC PATCH] tracing: fix return value in __ftrace_event_enable_disable for TRACE_REG_UNREGISTER

[RFC PATCH] tracing: fix return value in __ftrace_event_enable_disable for TRACE_REG_UNREGISTER

[Gabriele Paoloni]

When __ftrace_event_enable_disable invokes the class callback to
unregister the event, the return value is not reported up to the
caller, hence leading to event unregister failures being silently
ignored.

This patch assigns the ret variable to the invocation of the
event unregister callback, so that its return value is stored
and reported to the caller.

Signed-off-by: Gabriele Paoloni <gpaoloni@redhat.com>
---
Sending this as RFC since I am not sure if checking the ret
value is really needed.
I have been mainly driven by the implementation of
disable_trace_kprobe, disable_trace_fprobe,
tracepoint_probe_unregister, disable_trace_eprobe that can
return an error.

 kernel/trace/trace_events.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 513de9ceb80e..8d92b271ce0d 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -790,7 +790,7 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
 				clear_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, &file->flags);
 			}
 
-			call->class->reg(call, TRACE_REG_UNREGISTER, file);
+			ret = call->class->reg(call, TRACE_REG_UNREGISTER, file);
 		}
 		/* If in SOFT_MODE, just set the SOFT_DISABLE_BIT, else clear it */
 		if (file->flags & EVENT_FILE_FL_SOFT_MODE)
-- 
2.48.1

Re: [RFC PATCH] tracing: fix return value in __ftrace_event_enable_disable for TRACE_REG_UNREGISTER

[Masami Hiramatsu]

On Fri, 14 Mar 2025 13:57:25 +0100
Gabriele Paoloni <gpaoloni@redhat.com> wrote:

> When __ftrace_event_enable_disable invokes the class callback to
> unregister the event, the return value is not reported up to the
> caller, hence leading to event unregister failures being silently
> ignored.
> 
> This patch assigns the ret variable to the invocation of the
> event unregister callback, so that its return value is stored
> and reported to the caller.

Just out of curiosity, have you saw such issue? I think
event unregister should be succeeded or it warns the
fault.

> 
> Signed-off-by: Gabriele Paoloni <gpaoloni@redhat.com>
> ---
> Sending this as RFC since I am not sure if checking the ret
> value is really needed.
> I have been mainly driven by the implementation of
> disable_trace_kprobe, disable_trace_fprobe,
> tracepoint_probe_unregister, disable_trace_eprobe that can
> return an error.
> 
>  kernel/trace/trace_events.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 513de9ceb80e..8d92b271ce0d 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -790,7 +790,7 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
>  				clear_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, &file->flags);
>  			}
>  
> -			call->class->reg(call, TRACE_REG_UNREGISTER, file);
> +			ret = call->class->reg(call, TRACE_REG_UNREGISTER, file);

This is not enough. As same as enable failure, this function needs to handle
this error to report it and break.

Thank you,

>  		}
>  		/* If in SOFT_MODE, just set the SOFT_DISABLE_BIT, else clear it */
>  		if (file->flags & EVENT_FILE_FL_SOFT_MODE)
> -- 
> 2.48.1
> 
> 


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

Re: [RFC PATCH] tracing: fix return value in __ftrace_event_enable_disable for TRACE_REG_UNREGISTER

[Steven Rostedt]

On Tue, 18 Mar 2025 11:07:00 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > --- a/kernel/trace/trace_events.c
> > +++ b/kernel/trace/trace_events.c
> > @@ -790,7 +790,7 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
> >  				clear_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, &file->flags);
> >  			}
> >  
> > -			call->class->reg(call, TRACE_REG_UNREGISTER, file);
> > +			ret = call->class->reg(call, TRACE_REG_UNREGISTER, file);  
> 
> This is not enough. As same as enable failure, this function needs to handle
> this error to report it and break.

Perhaps all we should do here is:

			WARN_ON_ONCE(ret);

-- Steve

Re: [RFC PATCH] tracing: fix return value in __ftrace_event_enable_disable for TRACE_REG_UNREGISTER

[Gabriele Paoloni]

On Wed, Mar 19, 2025 at 10:13 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 18 Mar 2025 11:07:00 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
>
> > > --- a/kernel/trace/trace_events.c
> > > +++ b/kernel/trace/trace_events.c
> > > @@ -790,7 +790,7 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
> > >                             clear_bit(EVENT_FILE_FL_RECORDED_TGID_BIT, &file->flags);
> > >                     }
> > >
> > > -                   call->class->reg(call, TRACE_REG_UNREGISTER, file);
> > > +                   ret = call->class->reg(call, TRACE_REG_UNREGISTER, file);
> >
> > This is not enough. As same as enable failure, this function needs to handle
> > this error to report it and break.
>
> Perhaps all we should do here is:
>
>                         WARN_ON_ONCE(ret);

Yes I was trying to address the request from Masami however,
if we look at the error path of TRACE_REG_REGISTER what happens is:

ret = call->class->reg(call, TRACE_REG_REGISTER, file);
if (ret) {
   if (cmd)
       tracing_stop_cmdline_record();
    if (tgid)
       tracing_stop_tgid_record();
    pr_info("event trace: Could not enable event "
       "%s\n", trace_event_name(call));
    break;

In this case it makes sense since it will undo what was done before
enabling the trace event. However I don't think it would make sense
to invoke tracing_start_cmdline_record and/or tracing_start_tgid_record
if the unregister callbacks fails since the event could be in a sort of
unknown state (i.e. we cannot trust the event to still be enabled).

Also it is probably not appropriate to log the event using exclusively

         pr_info("event trace: Could not disable event "
                 "%s\n", trace_event_name(call));

So I would agree with Steven or else

      if (ret)
           pr_warn("event trace: Could not disable event "
                   "%s\n", trace_event_name(call));

Many thanks for the review
Gab
>
> -- Steve
>