On Mar 12 16:30, Corinna Vinschen via Cygwin wrote: > On Mar 11 12:32, Christian Franke via Cygwin wrote: > > Corinna Vinschen via Cygwin wrote: > > > It's not quite clear to me why signal handling should be broken if > > > setcontext is used inside a signal handler. The incyg flag is false > > > when running the signal handler and that's correct. Theoretically a > > > running signal handler is not different from other process code. > > > > > > Do you have an STC, by any chance? > > > > The attached testcase should test the following use cases of setcontext: > > - call from regular user space > > - call from a signal handler interrupting user space > > - call from a signal handler interrupting a system call > > > > It works as expected ... until the signal count reaches 256. Then signals > > are again only delivered from inside of a system call. > > [...] > > Interesting... Hmm... is there some 8-bit counter which overflows and then > > stucks at 0xff or 0x00? > > It's a kind of stack overflow. Kind of, because it's not the normal > thread stack, but a special signal stack in the _cygtls area. > > When interrupting a running thread to call a signal handler, the context > of the thread is changed to restart execution in an assembler function > called sigdelayed(). The original IP of the thread is pushed on the > aforementioned signal stack. Sigdelayed() calls the signal handler. On > return it pops the original IP from the signal stack and continues the > thread. > > Now guess what happens if the signal handler bails out with longjmp or > setcontext/swapcontext. > > The signal handler never returns to the sigdelayed() function, the > original address is never poped from the signal stack, and the signal > stack has a max. size of 256 address entries... > > Theoretically, a small update to sigdelayed() would fix the issue: ather > then poing the original IP from the signal stack after calling the > handler, it should pop the IP prior to calling the handler. That would > avoid filling up the signal stack when long-jumping out of the signal > handler. It should store the IP in one of the callee-saved registers. > %r13 is unused in sigdelayed so far. > > However, even if we do this, there's still the problem that sigdelayed() > itself takes space on the stack. If you longjmp/setcontext out of the > handler, the thread's normal stack will fill up with dead storage of the > sigdelayed() function, and there's no way out of this trap. We can't > restore the stack before the handler returns. > > So either way, at one point you get a stack overflow one way or the > other. > > The signal stack overflow is actually rather harmless in comparison > to a real stack overflow. > > If you have any idea how to avoid the real stack overflow, I'd be > all ears.
Looks like this isn't really a problem with setcontext. It always corrects the stack pointer as well. Apparently I haven't thought long enough about this. I have a patch for sigdelayed() in the loop, stay tuned. Corinna -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
