Fetch the sigfastblock value in syscalls that wait for signals
We have seen several cases of processes which have become "stuck" in kern_sigsuspend(). When this occurs, the kernel's td_sigblock_val is set to 0x10 (one block outstanding) and the userspace copy of the word is set to 0 (unblocked). Because the kernel's cached value shows that signals are blocked, kern_sigsuspend() blocks almost all signals, which means the process hangs indefinitely in sigsuspend(). It is not entirely clear what is causing this condition to occur. However, it seems to make sense to add some protection against this case by fetching the latest sigfastblock value from userspace for syscalls which will sleep waiting for signals. Here, the change is applied to kern_sigsuspend() and kern_sigtimedwait(). Reviewed by: kib Sponsored by: Netflix Differential Revision: https://reviews.freebsd.org/D29225
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@ -1268,6 +1268,9 @@ kern_sigtimedwait(struct thread *td, sigset_t waitset, ksiginfo_t *ksi,
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ets.tv_nsec = 0;
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traced = false;
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/* Ensure the sigfastblock value is up to date. */
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sigfastblock_fetch(td);
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if (timeout != NULL) {
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if (timeout->tv_nsec >= 0 && timeout->tv_nsec < 1000000000) {
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timevalid = 1;
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@ -1527,6 +1530,9 @@ kern_sigsuspend(struct thread *td, sigset_t mask)
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struct proc *p = td->td_proc;
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int has_sig, sig;
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/* Ensure the sigfastblock value is up to date. */
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sigfastblock_fetch(td);
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/*
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* When returning from sigsuspend, we want
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* the old mask to be restored after the
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