Re: [BUG/RFC 1/2] arm64/ftrace,bpf: Fix partial regs after bpf_prog_run

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 4 Jan 2026 22:34:15 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > This looks a bit grotty to me and presumably other architectures would
> > need similar treatement. Wouldn't it be cleaner to reuse the existing
> > API instead? For example, by calling ftrace_regs_set_instruction_pointer()
> > and ftrace_regs_set_return_value() to update the relevant registers from
> > the core code?  
> 
> I agreed with using the generic APIs. Also, ftrace_partial_regs_fix() is
> not self-explained. Maybe ftrace_regs_set_by_regs()?

Or perhaps: ftrace_partial_regs_update() where you call it if you need to
update the regs.

/*
 * ftrace_partial_regs_update - update the original ftrace_regs from regs
 * @fregs: The ftrace_regs to update from @regs
 * @regs: The partial regs from ftrace_partial_regs() that was updated
 *
 * Some architectures have the partial regs living in the ftrace_regs
 * structure, whereas other architectures need to make a different copy
 * of the @regs. If a partial @regs is retrieved by ftrace_partial_regs() and
 * if the code using @regs updates a field (like the instruction pointer or
 * stack pointer) it may need to propagate that change to the original @fregs
 * it retrieved the partial @regs from. Use this function to guarantee that
 * update happens.
 */
static __always_inline void
ftrace_partial_regs_update(const struct ftrace_regs *fregs, struct pt_regs *regs) {
	struct __arch_ftrace_regs *afregs = arch_ftrace_regs(fregs);

	if (afregs->pc != regs->pc) {
		afregs->pc = regs->pc;
		afregs->regs[0] = regs->regs[0];
	}
}

-- Steve