check that there's no panic if user process tries to write >= MAXVA

user/usertests.c

@@ -2228,6 +2228,30 @@ kernmem(char *s)
   }
 }
 
+// user code should not be able to write to addresses above MAXVA.
+void
+MAXVAplus(char *s)
+{
+  volatile uint64 a = MAXVA;
+  for( ; a != 0; a <<= 1){
+    int pid;
+    pid = fork();
+    if(pid < 0){
+      printf("%s: fork failed\n", s);
+      exit(1);
+    }
+    if(pid == 0){
+      *(char*)a = 99;
+      printf("%s: oops wrote %x\n", s, a);
+      exit(1);
+    }
+    int xstatus;
+    wait(&xstatus);
+    if(xstatus != -1)  // did kernel kill child?
+      exit(1);
+  }
+}
+
 // if we run the system out of memory, does it clean up the last
 // failed allocation?
 void
@@ -2802,6 +2826,7 @@ main(int argc, char *argv[])
     void (*f)(char *);
     char *s;
   } tests[] = {
+    {MAXVAplus, "MAXVAplus"},
     {manywrites, "manywrites"},
     {execout, "execout"},
     {copyin, "copyin"},