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

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

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?

Thanks,

> + *
> + * 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.
> + * On the function exit, onlu registers which is used for return values
> + * are restored.
> + *
> + * 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>