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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Mar 20, 2024 at 10:44:30AM -0700, Andrii Nakryiko wrote: > On Wed, Mar 20, 2024 at 8:30 AM Oleg Nesterov 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. > >