c18n: Remove libsys's reference to a libc function

Previously, lib/c18n/libsys's sigaction calls a libc function which in
turn calls the RTLD hook. This is broken but was undetected because c18n
was accidentally always disabled in the c18n-version of libsys.

lib/libc/include/libc_private.h

@@ -396,7 +396,7 @@ int __strerror_rl(int errnum, char *strerrbuf, size_t buflen,
 
 #ifdef CHERI_LIB_C18N
 __pid_t		__sys_rfork(int);
-int	sigaction_c18n(int, const struct sigaction *, struct sigaction *);
+int	_rtld_sigaction(int, const struct sigaction *, struct sigaction *);
 #endif
 
 #endif /* _LIBC_PRIVATE_H_ */

lib/libc/sys/sigaction.c

@@ -35,19 +35,6 @@
 
 __weak_reference(__sys_sigaction, __sigaction);
 __weak_reference(sigaction, __libc_sigaction);
-#ifdef CHERI_LIB_C18N
-/*
- * This weak symbol will always be resolved at runtime.
- */
-#pragma weak _rtld_sigaction
-int _rtld_sigaction(int, const struct sigaction *, struct sigaction *);
-
-int
-sigaction_c18n(int sig, const struct sigaction *act, struct sigaction *oact)
-{
-	return (_rtld_sigaction(sig, act, oact));
-}
-#endif
 
 #pragma weak sigaction
 int

lib/libsys/interposing_table.c

@@ -32,6 +32,19 @@
 #include <sys/types.h>
 #include "libc_private.h"
 
+#ifdef CHERI_LIB_C18N
+/*
+ * XXX: A wrapper is needed to redirect the call because directly referencing
+ * _rtld_sigaction as a function pointer causes it to not be wrapped in a
+ * trampoline at the moment.
+ */
+int
+_rtld_sigaction_wrapper(int sig, const struct sigaction *oact,
+    struct sigaction *act) {
+	return (_rtld_sigaction(sig, oact, act));
+}
+#endif
+
 #define	SLOT(a, b) \
 	[INTERPOS_##a] = (interpos_func_t)b
 static interpos_func_t __libsys_interposing[INTERPOS_MAX] = {
@@ -57,7 +70,7 @@ static interpos_func_t __libsys_interposing[INTERPOS_MAX] = {
 	SLOT(sendto, __sys_sendto),
 	SLOT(setcontext, __sys_setcontext),
 #ifdef CHERI_LIB_C18N
-	SLOT(sigaction, sigaction_c18n),
+	SLOT(sigaction, _rtld_sigaction_wrapper),
 #else
 	SLOT(sigaction, __sys_sigaction),
 #endif