Re: [PATCH v14 05/19] function_graph: Pass ftrace_regs to retfunc

Masami Hiramatsu
2024-10-01T23:24:46Z

On Mon, 30 Sep 2024 15:03:02 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue, 17 Sep 2024 11:08:48 +0100
> Will Deacon <will@kernel.org> wrote:
>
> > > > @@ -787,6 +789,9 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
> > > > }
> > > >
> > > > trace.rettime = trace_clock_local();
> > > > + if (fregs)
> > > > + ftrace_regs_set_instruction_pointer(fregs, ret);
> >
> > Where does the instruction pointer get used after this? The arm64
> > 'return_to_handler' function doesn't look at it when we return.
>
> It's for the hooks to the return instruction. kretprobes will start using

not kretprobes, but fprobe. kretprobes continue using rethook.

> function graph tracer to hook to a return of a function (via fprobes), and
> the callbacks will need access to the return pointer. The callbacks get
> passed the ftrace_regs, and this is how they can see what the function is
> returning to. For example, BPF programs will need this.
>
> So it's not needed for the infrastructure, only the callbacks that hook to
> it.

Yes, it will be used for showing where to return in the fprobe exit event.
More specifically, in the fprobe_return()@kernel/trace/fprobe.c in PATCH 13/19,
it is extracted from fregs.

+static void fprobe_return(struct ftrace_graph_ret *trace,
+ struct fgraph_ops *gops,
+ struct ftrace_regs *fregs)
+{
+ unsigned long *fgraph_data = NULL;
+ unsigned long ret_ip;
+ unsigned long val;
+ struct fprobe *fp;
+ int size, curr;
+ int size_words;
+
+ fgraph_data = (unsigned long *)fgraph_retrieve_data(gops->idx, &size);
+ if (WARN_ON_ONCE(!fgraph_data))
return;
+ size_words = SIZE_IN_LONG(size);
+ ret_ip = ftrace_regs_get_instruction_pointer(fregs);
+

Thank you,

>
> -- Steve

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