sched/signal: Add logic and an interface to determin if a signal can be caught or ignored. sigaction now correctly returns EINVAL on any attempt to catch or ignore such signals (only SIGKILL for now and only if CONFIG_SIG_DEFAULT=y).

drivers/serial/Kconfig

@@ -141,10 +141,10 @@ config TTY_SIGINT
 		devices.  May be enabled for other serial devices using the ISIG bit
 		in the Termios c_lflag.
 
-		REVISIT:  This implementation is non-standard.  The c_lflag ISIG bit
-		normally enables/disables INTR, QUIT, SUSP, and DSUSP character
-		processing.  The relationship between these names, standard signals,
-		and typical key presses are as follows:
+		REVISIT:  This implementation is compliant but incomplete.  The
+		c_lflag ISIG bit normally enables/disables INTR, QUIT, SUSP, and
+		DSUSP character processing.  The relationship between these names,
+		standard signals, and typical key presses are as follows:
 
 		  INTR   SIGINT   Ctrl-C  ETX(0x03) Interrupt
 		  KILL   SIGKILL  Ctrl-U  NAK(0x15) Kill

sched/signal/sig_action.c

@@ -137,8 +137,8 @@ static FAR sigactq_t *nxsig_alloc_action(void)
  * Assumptions:
  *
  * POSIX Compatibility:
- * - There are no default actions so the special value SIG_DFL is treated
- *   like SIG_IGN.
+ * - If CONFIG_SIG_DEFAULT is not defined, then there are no default actions
+ *   so the special value SIG_DFL is treated like SIG_IGN.
  * - All sa_flags in struct sigaction of act input are ignored (all
  *   treated like SA_SIGINFO). The one exception is if CONFIG_SCHED_CHILD_STATUS
  *   is defined; then SA_NOCLDWAIT is supported but only for SIGCHLD
@@ -168,6 +168,19 @@ int sigaction(int signo, FAR const struct sigaction *act,
       return ERROR;
     }
 
+#ifdef CONFIG_SIG_DEFAULT
+  /* Check if the user is trying to catch or ignore a signal that cannot be
+   * caught or ignored.
+   */
+
+  if (act != NULL &&
+      (act->sa_handler != SIG_DFL && !nxsig_iscatchable(signo)))
+    {
+      set_errno(EINVAL);
+      return ERROR;
+    }
+#endif
+
   /* Find the signal in the signal action queue */
 
   sigact = nxsig_find_action(group, signo);

sched/signal/sig_default.c

@@ -48,6 +48,14 @@
 
 #include "signal/signal.h"
 
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/* Bit definitions for the struct nxsig_defaction_s 'flags' field */
+
+#define SIG_FLAG_NOCATCH (1 << 0)  /* Signal cannot be caught or ignored */
+
 /****************************************************************************
  * Private Types
  ****************************************************************************/
@@ -57,6 +65,7 @@
 struct nxsig_defaction_s
 {
   uint8_t       signo;   /* Signal number.  Range 1..MAX_SIGNO */
+  uint8_t       flags;   /* See SIG_FLAG_ definitions */
   _sa_handler_t action;  /* Default signal action */
 };
 
@@ -91,19 +100,19 @@ static void nxsig_setup_default_action(FAR struct task_group_s *group,
 static const struct nxsig_defaction_s g_defactions[] =
 {
 #ifdef CONFIG_SIG_SIGUSR1_ACTION
-   { SIGUSR1, nxsig_abnormal_termination },
+   { SIGUSR1, 0,                nxsig_abnormal_termination },
 #endif
 #ifdef CONFIG_SIG_SIGUSR2_ACTION
-   { SIGUSR2, nxsig_abnormal_termination },
+   { SIGUSR2, 0,                nxsig_abnormal_termination },
 #endif
 #ifdef CONFIG_SIG_SIGALRM_ACTION
-   { SIGALRM, nxsig_abnormal_termination },
+   { SIGALRM, 0,                nxsig_abnormal_termination },
 #endif
 #ifdef CONFIG_SIG_SIGPOLL_ACTION
-   { SIGPOLL, nxsig_abnormal_termination },
+   { SIGPOLL, 0,                nxsig_abnormal_termination },
 #endif
-   { SIGINT,  nxsig_abnormal_termination },
-   { SIGKILL, nxsig_abnormal_termination }
+   { SIGINT,  0,                nxsig_abnormal_termination },
+   { SIGKILL, SIG_FLAG_NOCATCH, nxsig_abnormal_termination }
 };
 
 #define NACTIONS (sizeof(g_defactions) / sizeof(struct nxsig_defaction_s))
@@ -272,6 +281,42 @@ bool nxsig_isdefault(FAR struct tcb_s *tcb, int signo)
   return ret < 0 ? false : (bool)ret;
 }
 
+/****************************************************************************
+ * Name: nxsig_iscatchable
+ *
+ * Description:
+ *   Return true if the specified signal can be caught or ignored.
+ *
+ * Input Parameters:
+ *   signo - The signal number to be queried
+ *
+ * Returned Value:
+ *   Zero (OK) is returned on success; A negated errno value is returned
+ *   on failure.
+ *
+ ****************************************************************************/
+
+bool nxsig_iscatchable(int signo)
+{
+  int i;
+
+  /* Search the default action table for the entry associated with this
+   * signal.
+   */
+
+  for (i = 0; i < NACTIONS; i++)
+    {
+      if (g_defactions[i].signo == signo)
+        {
+          return (g_defactions[i].flags & SIG_FLAG_NOCATCH) == 0;
+        }
+    }
+
+  /* If it is not in the table, then it is catchable */
+
+  return true;
+}
+
 /****************************************************************************
  * Name: nxsig_default
  *

sched/signal/signal.h

@@ -174,6 +174,7 @@ void               nxsig_release_action(FAR sigactq_t *sigact);
 
 #ifdef CONFIG_SIG_DEFAULT
 bool               nxsig_isdefault(FAR struct tcb_s *tcb, int signo);
+bool               nxsig_iscatchable(int signo);
 _sa_handler_t      nxsig_default(FAR struct tcb_s *tcb, int signo,
                                  bool defaction);
 int                nxsig_default_initialize(FAR struct tcb_s *tcb);