Re: [RFC PATCH v2 01/31] tracing: Add a comment about ftrace_regs definition

[Mark Rutland]

On Thu, Nov 09, 2023 at 08:14:52AM +0900, Masami Hiramatsu wrote:
> On Wed,  8 Nov 2023 23:24:32 +0900
> "Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > 
> > To clarify what will be expected on ftrace_regs, add a comment to the
> > architecture independent definition of the ftrace_regs.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > ---
> >  Changes in v2:
> >   - newly added.
> > ---
> >  include/linux/ftrace.h |   25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> > 
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index e8921871ef9a..b174af91d8be 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -118,6 +118,31 @@ extern int ftrace_enabled;
> >  
> >  #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
> >  
> > +/**
> > + * ftrace_regs - ftrace partial/optimal register set
> > + *
> > + * ftrace_regs represents a group of registers which is used at the
> > + * function entry and exit. There are three types of registers.
> > + *
> > + * - Registers for passing the parameters to callee, including the stack
> > + *   pointer. (e.g. rcx, rdx, rdi, rsi, r8, r9 and rsp on x86_64)
> > + * - Registers for passing the return values to caller.
> > + *   (e.g. rax and rdx on x86_64)
> > + * - Registers for hooking the function return including the frame pointer
> > + *   (the frame pointer is architecture/config dependent)
> > + *   (e.g. rbp and rsp for x86_64)
> 
> Oops, I found the program counter/instruction pointer must be saved too.
> This is used for live patching. One question is that if the IP is modified
> at the return handler, what should we do? Return to the specified address?

I'm a bit confused here; currently we use fgraph_ret_regs for function returns,
are we going to replace that with ftrace_regs?

I think it makes sense for the PC/IP to be the address the return handler will
eventually return to (and hence allowing it to be overridden), but that does
mean we'll need to go recover the return address *before* we invoke any return
handlers.

Thanks,
Mark.