always terminate by SIGABRT when abort is called

Linux makes this surprisingly difficult, but it can be done. the trick
here is using the fact that we control the implementation of sigaction
to prevent changing the disposition of SIGABRT to anything but SIG_DFL
after abort has tried and failed to terminate the process simply by
calling raise(SIGABRT).

src/exit/abort.c

@@ -3,11 +3,30 @@
 #include "syscall.h"
 #include "pthread_impl.h"
 #include "atomic.h"
+#include "libc.h"
+#include "ksigaction.h"
+
+__attribute__((__visibility__("hidden")))
+volatile int __abort_lock[1];
 
 _Noreturn void abort(void)
 {
 	raise(SIGABRT);
+
+	/* If there was a SIGABRT handler installed and it returned, or if
+	 * SIGABRT was blocked or ignored, take an AS-safe lock to prevent
+	 * sigaction from installing a new SIGABRT handler, uninstall any
+	 * handler that may be present, and re-raise the signal to generate
+	 * the default action of abnormal termination. */
 	__block_all_sigs(0);
+	LOCK(__abort_lock);
+	__syscall(SYS_rt_sigaction, SIGABRT,
+		&(struct k_sigaction){.handler = SIG_DFL}, 0, _NSIG/8);
+	__syscall(SYS_tkill, __pthread_self()->tid, SIGABRT);
+	__syscall(SYS_rt_sigprocmask, SIG_UNBLOCK,
+		&(long[_NSIG/(8*sizeof(long))]){1UL<<(SIGABRT-1)}, 0, _NSIG/8);
+
+	/* Beyond this point should be unreachable. */
 	a_crash();
 	raise(SIGKILL);
 	_Exit(127);

src/signal/sigaction.c

@@ -6,6 +6,11 @@
 #include "libc.h"
 #include "ksigaction.h"
 
+volatile int dummy_lock[1] = { 0 };
+
+__attribute__((__visibility__("hidden")))
+weak_alias(dummy_lock, __abort_lock);
+
 static int unmask_done;
 static unsigned long handler_set[_NSIG/(8*sizeof(long))];
 
@@ -17,6 +22,7 @@ void __get_handler_set(sigset_t *set)
 int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigaction *restrict old)
 {
 	struct k_sigaction ksa, ksa_old;
+	unsigned long set[_NSIG/(8*sizeof(long))];
 	if (sa) {
 		if ((uintptr_t)sa->sa_handler > 1UL) {
 			a_or_l(handler_set+(sig-1)/(8*sizeof(long)),
@@ -36,19 +42,30 @@ int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigact
 				unmask_done = 1;
 			}
 		}
+		/* Changing the disposition of SIGABRT to anything but
+		 * SIG_DFL requires a lock, so that it cannot be changed
+		 * while abort is terminating the process after simply
+		 * calling raise(SIGABRT) failed to do so. */
+		if (sa->sa_handler != SIG_DFL && sig == SIGABRT) {
+			__block_all_sigs(&set);
+			LOCK(__abort_lock);
+		}
 		ksa.handler = sa->sa_handler;
 		ksa.flags = sa->sa_flags | SA_RESTORER;
 		ksa.restorer = (sa->sa_flags & SA_SIGINFO) ? __restore_rt : __restore;
 		memcpy(&ksa.mask, &sa->sa_mask, sizeof ksa.mask);
 	}
-	if (syscall(SYS_rt_sigaction, sig, sa?&ksa:0, old?&ksa_old:0, sizeof ksa.mask))
-		return -1;
-	if (old) {
+	int r = __syscall(SYS_rt_sigaction, sig, sa?&ksa:0, old?&ksa_old:0, sizeof ksa.mask);
+	if (sig == SIGABRT && sa && sa->sa_handler != SIG_DFL) {
+		UNLOCK(__abort_lock);
+		__restore_sigs(&set);
+	}
+	if (old && !r) {
 		old->sa_handler = ksa_old.handler;
 		old->sa_flags = ksa_old.flags;
 		memcpy(&old->sa_mask, &ksa_old.mask, sizeof ksa_old.mask);
 	}
-	return 0;
+	return __syscall_ret(r);
 }
 
 int __sigaction(int sig, const struct sigaction *restrict sa, struct sigaction *restrict old)