main now keeps all four words of the guest signal mask. A handler's sa_mask above signal 32 is added too, and SIGKILL and SIGSTOP stay unblocked, as sigprocmask leaves them.
A guest signal handler ran with the mask it was raised under, so a
handler that raised its own signal again re-entered itself at once,
sigprocmask inside the handler did not report the signal or the
handler's sa_mask as blocked, and SA_RESETHAND left the handler
installed for the next delivery.
FreeBSD 11.4 forms the handler's frame and then, in postsig_done()
(sys/kern/kern_sig.c), blocks the handler's sa_mask and, unless
SA_NODEFER is set, the signal itself, and resets the disposition to
SIG_DFL when SA_RESETHAND is set; sigreturn restores the previous mask,
so a signal raised meanwhile stays pending until the handler returns.
sigaction keeps reporting SA_RESETHAND afterwards, since sigdflt() does
not clear ps_sigreset.
The handler now runs with its signal (unless SA_NODEFER) and its mask
added to the guest mask, which is restored when it returns, and pending
signals that the restored mask unblocks are delivered when raise
returns. SA_RESETHAND resets the handler to the default before it runs.
Yunus Shaikh