diff --git a/include/core/defs.h b/include/core/defs.h
index 5095a9e..58aa1bc 100644
--- a/include/core/defs.h
+++ b/include/core/defs.h
@@ -530,6 +530,7 @@ int             proc_pgid(int pid);
 int             proc_setpgid(int pid, int pgid);
 int             proc_setsid(void);
 int             sigsend(int pid, int sig);
+int             sig_default_kills(int sig);
 int             proc_exists(int);
 int             proc_num(void);
 struct procinfo;
diff --git a/kernel/proc/proc.c b/kernel/proc/proc.c
index 99fa3c4..a562127 100644
--- a/kernel/proc/proc.c
+++ b/kernel/proc/proc.c
@@ -1508,6 +1508,34 @@ init_ignores(struct proc *p, int sig)
   return p == initproc && p->sig_handler[sig] == 0;
 }
 
+// Whether a signal's default action is to end the process.
+int
+sig_default_kills(int sig)
+{
+  return sig == SIGKILL || sig == SIGTERM || sig == SIGABRT ||
+         sig == SIGSEGV || sig == SIGFPE  || sig == SIGBUS  ||
+         sig == SIGILL  || sig == SIGINT  || sig == SIGQUIT ||
+         sig == SIGHUP  || sig == SIGPIPE || sig == SIGUSR1 ||
+         sig == SIGUSR2 || sig == SIGALRM;
+}
+
+// Whether sending `sig` ends `p` now: no handler, a default action that
+// kills, and **not blocked**. A blocked signal stays pending, which is the
+// whole of what blocking means -- and the contract signalfd is built on: a
+// program blocks SIGUSR1 and reads it from a descriptor instead. This used to
+// kill on arrival whatever the mask said. deliver_signals() applies the
+// default action if the signal is unblocked later, still pending.
+// SIGKILL cannot be blocked.
+static int
+sig_kills_now(struct proc *p, int sig)
+{
+  if(sig == SIGKILL)
+    return 1;
+  if(p->signal_mask & (1ULL << sig))
+    return 0;
+  return p->sig_handler[sig] == 0 && sig_default_kills(sig);
+}
+
 int
 sigsend(int pid, int sig)
 {
@@ -1524,12 +1552,7 @@ sigsend(int pid, int sig)
         return 0;           // silently dropped, as POSIX requires
       }
       p->pending_signals |= 1ULL << sig;
-      if(p->sig_handler[sig] == 0 &&
-         (sig == SIGKILL || sig == SIGTERM || sig == SIGABRT ||
-          sig == SIGSEGV || sig == SIGFPE  || sig == SIGBUS  ||
-          sig == SIGILL  || sig == SIGINT  || sig == SIGQUIT ||
-          sig == SIGHUP  || sig == SIGPIPE || sig == SIGUSR1 ||
-          sig == SIGUSR2 || sig == SIGALRM))
+      if(sig_kills_now(p, sig))
         p->killed = 1;
       if(sig == SIGKILL)
         p->killed = 1;
@@ -1573,12 +1596,7 @@ sigsend_pgrp(int pgid, int sig)
     if(init_ignores(p, sig))
       continue;              // see init_ignores(): this is the Ctrl-C case
     p->pending_signals |= 1ULL << sig;
-    if(p->sig_handler[sig] == 0 &&
-       (sig == SIGKILL || sig == SIGTERM || sig == SIGABRT ||
-        sig == SIGSEGV || sig == SIGFPE  || sig == SIGBUS  ||
-        sig == SIGILL  || sig == SIGINT  || sig == SIGQUIT ||
-        sig == SIGHUP  || sig == SIGPIPE || sig == SIGUSR1 ||
-        sig == SIGUSR2 || sig == SIGALRM)){
+    if(sig_kills_now(p, sig)){
       p->killed = 1;
       if(p->killsig == 0)
         p->killsig = sig;
diff --git a/kernel/sys/sys_sig.c b/kernel/sys/sys_sig.c
index 234c643..fb94294 100644
--- a/kernel/sys/sys_sig.c
+++ b/kernel/sys/sys_sig.c
@@ -191,8 +191,17 @@ deliver_signals(struct trapframe *tf)
         // applied by sigsend(); all that is left here is to stop it being
         // pending, which the old `continue` never did, so an ignored signal
         // was re-examined on the way out of every syscall forever after.
+        //
+        // Except for a signal that was blocked when it arrived: sigsend()
+        // left it pending without acting, and now that it is deliverable its
+        // default action happens here.
         if (myproc()->sig_handler[sig] <= 1) {
             myproc()->pending_signals &= ~(1ULL << sig);
+            if (myproc()->sig_handler[sig] == 0 && sig_default_kills(sig)) {
+                myproc()->killed = 1;
+                if (myproc()->killsig == 0)
+                    myproc()->killsig = sig;
+            }
             continue;
         }