First of all, let me repeat I am useless when it comes to the low-level or asm code. I can be easily wrong, and I am going to ask the questions. On 04/28, Al Viro wrote: > > It's actually worse than I thought - we can't just lift that check > to do_notify_resume() and be done with that. Suppose do_signal() does > get called on e.g. i386 or arm with !user_mode(regs). What'll happen next? > > We have TIF_SIGPENDING set in thread flags - otherwise we wouldn't get > there at all. OK, do_signal() doesn't do anything and returns. So does > do_notify_resume(). And we are back into the loop in asm glue, rereading > the thread flags (still unchanged), checking if anything is to be done > (yes, it is - TIF_SIGPENDING is still set), calling do_notify_resume(), > ad infinitum. > > Lifting the check into do_notify_resume() will not help at all, obviously. > > AFAICS we can get hit by that. Please look at 29a2e2836ff9ea65a603c89df217f4198973a74f x86-32: Fix endless loop when processing signals for kernel tasks > At least i386, arm and mips have > ret_from_fork going straight to "return from syscall" path, no checks for > return to user mode done. And process created by kernel_thread() will > go there. Looks like, the patch above fixes that. But, we call do_notify_resume() first, it would be nice to avoid this and remove the user_mode() check in do_signal() or lift into do_notify_resume(). > It's a narrow race, but AFAICS it's not impossible to hit - > guess the PID of kernel thread to be launched, send it a signal and hit > the moment before it gets to executing the payload. Yes. But note that the kernel threads run with all signals ignored. This is still possible, but a kernel thread should do allow_signal() and then call kernel_thread() (not kthread_create). Question. So far I know that on x86 do_notify_resume() && !user_mode() is only possible on 32bit system, and the possible callers are ret_from_fork or kernel_execve (if it fails). Plus, perhaps CONFIG_VM86 makes a difference? Could you please clarify? Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
