Re: [PATCH v10 01/36] tracing: Add a comment about ftrace_regs definition

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

On Thu, 23 May 2024 19:10:31 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue,  7 May 2024 23:08:12 +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>
> > Acked-by: Mark Rutland <mark.rutland@arm.com>
> > ---
> >  Changes in v8:
> >   - Update that the saved registers depends on the context.
> >  Changes in v3:
> >   - Add instruction pointer
> >  Changes in v2:
> >   - newly added.
> > ---
> >  include/linux/ftrace.h |   26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index 54d53f345d14..b81f1afa82a1 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -118,6 +118,32 @@ 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 call and return including the
> > + *   frame pointer (the frame pointer is architecture/config dependent)
> > + *   (e.g. rip, rbp and rsp for x86_64)
> > + *
> > + * Also, architecture dependent fields can be used for internal process.
> > + * (e.g. orig_ax on x86_64)
> > + *
> > + * On the function entry, those registers will be restored except for
> > + * the stack pointer, so that user can change the function parameters
> > + * and instruction pointer (e.g. live patching.)
> > + * On the function exit, only registers which is used for return values
> > + * are restored.
> 
> I wonder if we should also add a note about some architectures in some
> circumstances may store all pt_regs in ftrace_regs. For example, if an
> architecture supports FTRACE_WITH_REGS, it may pass the pt_regs within the
> ftrace_regs. If that is the case, then ftrace_get_regs() called on it will
> return a pointer to a valid pt_regs, or NULL if it is not supported or the
> ftrace_regs does not have a all the registers.

Agreed. That case also should be noted. Thanks for pointing!


> 
> -- Steve
> 
> 
> > + *
> > + * NOTE: user *must not* access regs directly, only do it via APIs, because
> > + * the member can be changed according to the architecture.
> > + */
> >  struct ftrace_regs {
> >  	struct pt_regs		regs;
> >  };
> 


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