Re: [PATCH v17 06/16] tracing: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs

[Will Deacon <will@kernel.org>]

On Wed, Oct 16, 2024 at 09:59:00AM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Add ftrace_partial_regs() which converts the ftrace_regs to pt_regs.
> This is for the eBPF which needs this to keep the same pt_regs interface
> to access registers.
> Thus when replacing the pt_regs with ftrace_regs in fprobes (which is
> used by kprobe_multi eBPF event), this will be used.
> 
> If the architecture defines its own ftrace_regs, this copies partial
> registers to pt_regs and returns it. If not, ftrace_regs is the same as
> pt_regs and ftrace_partial_regs() will return ftrace_regs::regs.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Acked-by: Florent Revest <revest@chromium.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: Albert Ou <aou@eecs.berkeley.edu>
> 
> ---
>  Changes in v14:
>   - Add riscv change.
>  Changes in v8:
>   - Add the reason why this required in changelog.
>  Changes from previous series: NOTHING, just forward ported.
> ---
>  arch/arm64/include/asm/ftrace.h |   11 +++++++++++
>  arch/riscv/include/asm/ftrace.h |   14 ++++++++++++++
>  include/linux/ftrace.h          |   17 +++++++++++++++++
>  3 files changed, 42 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index b5fa57b61378..d344c69eb01e 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -135,6 +135,17 @@ ftrace_regs_get_frame_pointer(const struct ftrace_regs *fregs)
>  	return arch_ftrace_regs(fregs)->fp;
>  }
>  
> +static __always_inline struct pt_regs *
> +ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
> +{
> +	memcpy(regs->regs, arch_ftrace_regs(fregs)->regs, sizeof(u64) * 9);

Since ftrace_regs::regs is an 'unsigned long regs[9]' can we just use
sizeof() on that instead of hard-coding the length of the array here?

Will