Re: [PATCH v18 01/17] fgraph: Pass ftrace_regs to entryfunc

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

On Thu, 31 Oct 2024 15:53:24 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sat, 26 Oct 2024 13:35:30 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > Pass ftrace_regs to the fgraph_ops::entryfunc(). If ftrace_regs is not
> > available, it passes a NULL instead. User callback function can access
> > some registers (including return address) via this ftrace_regs.
> > 
> > Note that the ftrace_regs can be NULL when the arch does NOT define:
> > HAVE_DYNAMIC_FTRACE_WITH_ARGS or HAVE_DYNAMIC_FTRACE_WITH_REGS.
> > More specifically, if HAVE_DYNAMIC_FTRACE_WITH_REGS is defined but
> > not the HAVE_DYNAMIC_FTRACE_WITH_ARGS, and the ftrace ops used to
> > register the function callback does not set FTRACE_OPS_FL_SAVE_REGS.
> > In this case, ftrace_regs can be NULL in user callback.
> 
> If HAVE_DYNAMIC_FTRACE_WITH_REGS is defined but not
> HAVE_DYNAMIC_FTRACE_WITH_ARGS is not, then the callback will have regs defined.
> 
> > @@ -977,7 +980,7 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
> >  
> >  static struct ftrace_ops graph_ops = {
> >  	.func			= ftrace_graph_func,
> > -	.flags			= FTRACE_OPS_GRAPH_STUB,
> > +	.flags			= FTRACE_OPS_GRAPH_STUB | FTRACE_OPS_FL_SAVE_ARGS,
> 
> Enabling FTRACE_OPS_FL_SAVE_ARGS will pass full regs in that case. Are you
> just saying in the change log that this is what you did? As it currently
> reads, it sounds like a fgraph user needs to add FTRACE_OPS_FL_SAVE_REGS??

Ah, good catch! It should put the flag only when HAVE_DYNAMIC_FTRACE_WITH_ARGS
is enabled.

static struct ftrace_ops graph_ops = {
	.func			= ftrace_graph_func,
#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
	.flags			= FTRACE_OPS_GRAPH_STUB | FTRACE_OPS_FL_SAVE_ARGS,
#elif defined(CONFIG_DYNAMIC_FTRACE_WITH_ARGS)
	.flags			= FTRACE_OPS_GRAPH_STUB | FTRACE_OPS_FL_SAVE_REGS,
#else
	.flags			= FTRACE_OPS_GRAPH_STUB,
#endif

This will save fregs or regs or NULL according to the configuration.

> 
> -- Steve
> 
> 
> >  #ifdef FTRACE_GRAPH_TRAMP_ADDR
> >  	.trampoline		= FTRACE_GRAPH_TRAMP_ADDR,
> >  	/* trampoline_size is only needed for dynamically allocated tramps */
> > @@ -987,7 +990,8 @@ static struct ftrace_ops graph_ops = {
> >  void fgraph_init_ops(struct ftrace_ops *dst_ops,
> >  		     struct ftrace_ops *src_ops)
> >  {
> > -	dst_ops->flags = FTRACE_OPS_FL_PID | FTRACE_OPS_GRAPH_STUB;
> > +	dst_ops->flags = FTRACE_OPS_FL_PID | FTRACE_OPS_GRAPH_STUB |
> > +			 FTRACE_OPS_FL_SAVE_ARGS;
> >  
> >  #ifdef CONFIG_DYNAMIC_FTRACE
> >  	if (src_ops) {


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