Hi Jann, On 10/26/20 10:32 AM, Jann Horn wrote: > On Sat, Oct 24, 2020 at 2:53 PM Michael Kerrisk (man-pages) > <mtk.manpages@xxxxxxxxx> wrote: >> On 10/17/20 2:25 AM, Jann Horn wrote: >>> On Fri, Oct 16, 2020 at 8:29 PM Michael Kerrisk (man-pages) >>> <mtk.manpages@xxxxxxxxx> wrote: > [...] >>>> I'm not sure if I should write anything about this small UAPI >>>> breakage in BUGS, or not. Your thoughts? >>> >>> Thinking about it a bit more: Any code that relies on pause() or >>> epoll_wait() not restarting is buggy anyway, right? Because a signal >>> could also arrive directly before entering the syscall, while >>> userspace code is still executing? So one could argue that we're just >>> enlarging a preexisting race. (Unless the signal handler checks the >>> interrupted register state to figure out whether we already entered >>> syscall handling?) >> >> Yes, that all makes sense. >> >>> If userspace relies on non-restarting behavior, it should be using >>> something like epoll_pwait(). And that stuff only unblocks signals >>> after we've already past the seccomp checks on entry. >> >> Thanks for elaborating that detail, since as soon as you talked >> about "enlarging a preexisting race" above, I immediately wondered >> sigsuspend(), pselect(), etc. >> >> (Mind you, I still wonder about the effect on system calls that >> are normally nonrestartable because they have timeouts. My >> understanding is that the kernel doesn't restart those system >> calls because it's impossible for the kernel to restart the call >> with the right timeout value. I wonder what happens when those >> system calls are restarted in the scenario we're discussing.) > > Ah, that's an interesting edge case... I'm going to drop a FIXME into the page source so that there's a reminder of this issue in the next draft of the page, which I'm about to send out. [...] Thanks for checking the other pieces, Jann. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
