Re: [PATCH v11 02/11] tracing/probes: Add fprobe events for tracing function entry and exit.

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

On Thu, 8 Jun 2023 16:52:34 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 17 May 2023 19:59:04 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> 
> > Add fprobe events for tracing function entry and exit instead of kprobe
> > events. With this change, we can continue to trace function entry/exit
> > even if the CONFIG_KPROBES_ON_FTRACE is not available. Since
> > CONFIG_KPROBES_ON_FTRACE requires the CONFIG_DYNAMIC_FTRACE_WITH_REGS,
> > it is not available if the architecture only supports
> > CONFIG_DYNAMIC_FTRACE_WITH_ARGS. And that means kprobe events can not
> > probe function entry/exit effectively on such architecture.
> > But this can be solved if the dynamic events supports fprobe events.
> > 
> > The fprobe event is a new dynamic events which is only for the function
> > (symbol) entry and exit. This event accepts non register fetch arguments
> > so that user can trace the function arguments and return values.
> > 
> > The fprobe events syntax is here;
> > 
> >  f[:[GRP/][EVENT]] FUNCTION [FETCHARGS]
> >  f[MAXACTIVE][:[GRP/][EVENT]] FUNCTION%return [FETCHARGS]
> 
> I finally got around to look at these (I know you already queued them), but
> looking at the above, the "%return" is redundant.
> 
> > 
> > E.g.
> > 
> >  # echo 'f vfs_read $arg1'  >> dynamic_events
> >  # echo 'f vfs_read%return $retval'  >> dynamic_events
> >  # cat dynamic_events
> >  f:fprobes/vfs_read__entry vfs_read arg1=$arg1
> >  f:fprobes/vfs_read__exit vfs_read%return arg1=$retval
> 
> Can't we just have:
> 
>   f:fprobes/vfs_read__entry vfs_read arg1=$arg1
>   f:fprobes/vfs_read__exit vfs_read arg1=$retval
> 
> Where if "$retval" is specified, it automatically becomes a return? If
> anything else is specified, it errors out. That is, if $retval is
> specified, it becomes a return probe, as a return probe can only have
> $retval. If anything else is specified, it errors out if $retval is also
> specified.

Hmm, current implementation design doesn't allow that.
It parses the 'place' and 'args' sequencially because what 'args' is
available depends on the place.

> 
> Now if it's a void function, and you just want to make it a return then we
> can have your:
> 
>   f:fprobes/vfs_read__exit vfs_read%return
> 
> Thoughts?

But this sounds useful. Let me try to scan the argument to find $retval.

Thank you,

> 
> -- Steve
> 
> 
> 


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