Re: [PATCH RFC bpf-next 4/3] uprobe: ensure sys_uretprobe uses sysret

[Oleg Nesterov <oleg@redhat.com>]

On 03/20, Jiri Olsa wrote:
>
> are you ok if I squash the patches together

Yes, thanks, I am fine.

But lets discuss this change a bit more. So, with this poc we have the
(intentionally) oversimplified

	SYSCALL_DEFINE0(uretprobe)
	{
		struct pt_regs *regs = task_pt_regs(current);
		unsigned long __user *ax_and_ret = (unsigned long __user *)regs->sp + 2;
		unsigned long ip, err;

		ip = regs->ip;
		regs->orig_ax = -1;
		err = get_user(regs->ax, ax_and_ret);
		WARN_ON_ONCE(err);

		uprobe_handle_trampoline(regs);

		err = put_user(regs->ip, ax_and_ret);
		WARN_ON_ONCE(err);
		regs->ip = ip;

		return regs->ax;
	}

I have no idea what uprobe consumers / bpf programs can do, so let me ask:

	- uprobe_consumer's will see the "wrong" values of regs->cx/r11/sp
	  Is it OK? If not - easy to fix.

	- can uprobe_consumer change regs->cx/r11 ? If yes - easy to fix.

	- can uprobe_consumer change regs->sp ? If yes - easy to fix too,
	  but needs a separate check/code.

Oleg.