Re: [PATCHv5 perf/core 09/22] uprobes/x86: Add uprobe syscall to speed up uprobe

On Mon, Jul 14, 2025 at 11:28:01AM +0200, Peter Zijlstra wrote:

On Mon, Jul 14, 2025 at 05:39:15PM +0900, Masami Hiramatsu wrote:

quoted

quoted

+SYSCALL_DEFINE0(uprobe)
+{
+	struct pt_regs *regs = task_pt_regs(current);
+	unsigned long ip, sp, ax_r11_cx_ip[4];
+	int err;
+
+	/* Allow execution only from uprobe trampolines. */
+	if (!in_uprobe_trampoline(regs->ip))
+		goto sigill;
+

/*
 * When syscall from the trampoline, including a call to the trampoline
 * the stack will be shown as;
 *  regs->sp[0]: [rax]
 *          [1]: [r11]
 *          [2]: [rcx]
 *          [3]: [return-address] (probed address + sizeof(call-instruction))
 *
 * And the `&regs->sp[4]` should be the `sp` value when probe is hit.
 */

quoted

+	err = copy_from_user(ax_r11_cx_ip, (void __user *)regs->sp, sizeof(ax_r11_cx_ip));
+	if (err)
+		goto sigill;
+
+	ip = regs->ip;
+
+	/*
+	 * expose the "right" values of ax/r11/cx/ip/sp to uprobe_consumer/s, plus:
+	 * - adjust ip to the probe address, call saved next instruction address
+	 * - adjust sp to the probe's stack frame (check trampoline code)
+	 */
+	regs->ax  = ax_r11_cx_ip[0];
+	regs->r11 = ax_r11_cx_ip[1];
+	regs->cx  = ax_r11_cx_ip[2];
+	regs->ip  = ax_r11_cx_ip[3] - 5;
+	regs->sp += sizeof(ax_r11_cx_ip);
+	regs->orig_ax = -1;
+

Would not a structure be more natural?

/*
 * See uprobe syscall trampoline; the call to the trampoline will push
 * the return address on the stack, the trampoline itself then pushes
 * cx, r11 and ax.
 */
struct uprobe_syscall_args {
	unsigned long ax;
	unsigned long r11;
	unsigned long cx;
	unsigned long retaddr;
};

	err = copy_from_user(sys_args, (void __user *)regs->sp, sizeof(sys_args));
	if (err)
		goto sigill;

	ip = regs->ip;

	regs->ax  = sys_args->ax;
	regs->r11 = sys_args->r11;
	regs->cx  = sys_args->cx;
	regs->ip  = sys_args->retaddr - CALL_INSN_SIZE;
	regs->sp += sizeof(sys_args);

etc.. ?

I was mimicking sys_uretprobe, but using struct does seem better

thanks,
jirka