On Wed, Mar 20, 2024 at 10:44:30AM -0700, Andrii Nakryiko wrote:
> On Wed, Mar 20, 2024 at 8:30 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >
> > On 03/20, Oleg Nesterov wrote:
> > >
> > > 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.
> >
> > IOW. If answer is "yes" to all the questions above, then we probably need
> > something like
>
> yes to first, so ideally we fix registers to "correct" values
> (especially sp), but no to the last two (at least as far as BPF is
> concerned)
I think we should keep the same behaviour as it was for the trap,
so I think we should restore all registers and allow consumer to change it
jirka
>
> >
> > SYSCALL_DEFINE0(uretprobe)
> > {
> > struct pt_regs *regs = task_pt_regs(current);
> > unsigned long err, ip, sp, r11_cx_ax[3];
> >
> > err = copy_from_user(r11_cx_ax, (void __user*)regs->sp, sizeof(r11_cx_ax));
> > WARN_ON_ONCE(err);
> >
> > // Q1: apart from ax, do we really care?
> > // expose the "right" values of r11/cx/ax/sp to uprobe_consumer's
> > regs->r11 = r11_cx_ax[0];
> > regs->cx = r11_cx_ax[1];
> > regs->ax = r11_cx_ax[2];
> > regs->sp += sizeof(r11_cx_ax);
> > regs->orig_ax = -1;
> >
> > ip = regs->ip;
> > sp = regs->sp;
> >
> > uprobe_handle_trampoline(regs);
> >
> > // Q2: is it possible? do we care?
> > // uprobe_consumer has changed sp, we can do nothing,
> > // just return via iret.
> > if (regs->sp != sp)
> > return regs->ax;
> > regs->sp -= sizeof(r11_cx_ax);
> >
> > // Q3: is it possible? do we care?
> > // for the case uprobe_consumer has changed r11/cx
> > r11_cx_ax[0] = regs->r11;
> > r11_cx_ax[1] = regs->cx;
> >
> > // comment to explain this hack
> > r11_cx_ax[2] = regs->ip;
> > regs->ip = ip;
> >
> > err = copy_to_user((void __user*)regs->sp, r11_cx_ax, sizeof(r11_cx_ax));
> > WARN_ON_ONCE(err);
> >
> > // ensure sysret, see do_syscall_64()
> > regs->r11 = regs->flags;
> > regs->cx = regs->ip;
> >
> > return regs->ax;
> > }
> >
> > Oleg.
> >
