arch/: The saved return register state is available on the user stack. Thic commit reorders some logic so that certain, critical registers are preserved in the TCB. This does not make the logic 100% secure, but does prevent some obvious things. sched/signal/sig_delivery: Add a flag to the TCB to indicate that we are in a signal handler. Use this flag to assure that a there a never attempts to nest signal handling operations on a thread. This was guaranteed before but when locking of pre-emption during signal delivering was eliminated in a previous commit, there was a remote possibility of an attempt to do nested signal handling. This flag assures that there is only one signal handled at a time.

include/nuttx/sched.h

@@ -150,8 +150,9 @@
 #  define TCB_FLAG_SCHED_SPORADIC  (2 << TCB_FLAG_POLICY_SHIFT) /* Sporadic scheding policy */
 #  define TCB_FLAG_SCHED_OTHER     (3 << TCB_FLAG_POLICY_SHIFT) /* Other scheding policy */
 #define TCB_FLAG_CPU_LOCKED        (1 << 7) /* Bit 7: Locked to this CPU */
-#define TCB_FLAG_EXIT_PROCESSING   (1 << 8) /* Bit 8: Exitting */
-                                            /* Bits 9-15: Available */
+#define TCB_FLAG_SIGNAL_ACTION     (1 << 8) /* Bit 8: In a signal handler */
+#define TCB_FLAG_EXIT_PROCESSING   (1 << 9) /* Bit 9: Exitting */
+                                            /* Bits 10-15: Available */
 
 /* Values for struct task_group tg_flags */