overhaul implementation-internal signal protections

the new approach relies on the fact that the only ways to create
sigset_t objects without invoking UB are to use the sig*set()
functions, or from the masks returned by sigprocmask, sigaction, etc.
or in the ucontext_t argument to a signal handler. thus, as long as
sigfillset and sigaddset avoid adding the "protected" signals, there
is no way the application will ever obtain a sigset_t including these
bits, and thus no need to add the overhead of checking/clearing them
when sigprocmask or sigaction is called.

note that the old code actually *failed* to remove the bits from
sa_mask when sigaction was called.

the new implementations are also significantly smaller, simpler, and
faster due to ignoring the useless "GNU HURD signals" 65-1024, which
are not used and, if there's any sanity in the world, never will be
used.

src/thread/__rsyscall.c

@@ -56,8 +56,7 @@ int __rsyscall(int nr, long a, long b, long c, long d, long e, long f)
 	while ((i=rs.blocks))
 		__wait(&rs.blocks, 0, i, 1);
 
-	sigfillset(&set);
-	__libc_sigprocmask(SIG_BLOCK, &set, &set);
+	__syscall(SYS_rt_sigprocmask, SIG_BLOCK, (uint64_t[]){-1}, &set, 8);
 
 	if (!rs.init) {
 		struct sigaction sa = {
@@ -88,7 +87,7 @@ int __rsyscall(int nr, long a, long b, long c, long d, long e, long f)
 	}
 
 	/* Handle any lingering signals with no-op */
-	__libc_sigprocmask(SIG_UNBLOCK, &set, &set);
+	__syscall(SYS_rt_sigprocmask, SIG_SETMASK, &set, &set, 8);
 
 	/* Resume other threads' signal handlers and wait for them */
 	rs.hold = 0;

src/thread/pthread_create.c

@@ -34,7 +34,7 @@ void __pthread_unwind_next(struct __ptcb *cb)
 	if (!n) exit(0);
 
 	if (self->detached && self->map_base) {
-		__syscall(SYS_rt_sigprocmask, SIG_BLOCK, (long)(uint64_t[1]){-1},0,8);
+		__syscall(SYS_rt_sigprocmask, SIG_BLOCK, (uint64_t[]){-1},0,8);
 		__unmapself(self->map_base, self->map_size);
 	}
 
@@ -44,12 +44,8 @@ void __pthread_unwind_next(struct __ptcb *cb)
 static int start(void *p)
 {
 	struct pthread *self = p;
-	if (self->unblock_cancel) {
-		sigset_t set;
-		sigemptyset(&set);
-		sigaddset(&set, SIGCANCEL);
-		__libc_sigprocmask(SIG_UNBLOCK, &set, 0);
-	}
+	if (self->unblock_cancel)
+		__syscall(SYS_rt_sigprocmask, SIG_UNBLOCK, &SIGPT_SET, 0, 8);
 	pthread_exit(self->start(self->start_arg));
 	return 0;
 }
@@ -72,11 +68,7 @@ int pthread_create(pthread_t *res, const pthread_attr_t *attr, void *(*entry)(vo
 
 	if (!self) return ENOSYS;
 	if (!libc.threaded) {
-		sigset_t set;
-		sigemptyset(&set);
-		sigaddset(&set, SIGSYSCALL);
-		sigaddset(&set, SIGCANCEL);
-		__libc_sigprocmask(SIG_UNBLOCK, &set, 0);
+		__syscall(SYS_rt_sigprocmask, SIG_UNBLOCK, &SIGPT_SET, 0, 8);
 		libc.threaded = 1;
 	}