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

Steven Rostedt
2024-05-23T23:09:48Z

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.

-- 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;
> };