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

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

On Fri, 10 Nov 2023 11:11:31 +0000
Mark Rutland <mark.rutland@arm.com> wrote:

> 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@kernel.org> wrote:
> > 
> > > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > > 
> > > 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@kernel.org>
> > > ---
> > >  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?

Yes. It is limited and does not have APIs compatibility.

> 
> 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.

The actual return address has been recovered from shadow stack at first,
and callback the handlers. See __ftrace_return_to_handler() and
ftrace_pop_return_trace().
So it is easy to set it to the ftrace_regs :)

Thank you!

> 
> Thanks,
> Mark.


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