c18n: Do not attempt to get trusted stack when c18n is disabled

libexec/rtld-elf/aarch64/rtld_c18n_asm.S

@@ -30,6 +30,15 @@
 #include "rtld_c18n_machdep.h"
 #undef IN_ASM
 
+ENTRY(_rtld_unw_getcontext_epilogue)
+	/*
+	 * FIXME: llvm-libunwind specific ABI. This should be better specified.
+	 */
+	mov	c2, csp
+	str	c2, [c1]
+	RETURN
+END(_rtld_unw_getcontext_epilogue)
+
 ENTRY(_rtld_unw_setcontext_epilogue)
 	/*
 	 * FIXME: llvm-libunwind specific ABI. This should be better specified.

libexec/rtld-elf/rtld_c18n.c

@@ -827,6 +827,13 @@ resolve_untrusted_stk_impl(stk_table_index index)
 /*
  * Stack unwinding
  */
+/*
+ * Assembly functions that are tail-called when compartmentalisation is
+ * disabled.
+ */
+uintptr_t _rtld_unw_getcontext_epilogue(uintptr_t, void **);
+struct jmp_args _rtld_unw_setcontext_epilogue(struct jmp_args, void *, void **);
+
 static void *
 unwind_cursor()
 {
@@ -859,13 +866,21 @@ _rtld_setjmp(uintptr_t ret, void **buf)
 uintptr_t
 _rtld_unw_getcontext(uintptr_t ret, void **buf)
 {
+	if (!C18N_ENABLED) {
+		__attribute__((musttail))
+		return (_rtld_unw_getcontext_epilogue(ret, buf));
+	}
 	*buf = cheri_seal(unwind_cursor(), sealer_unwbuf);
 	return (ret);
 }
 
 uintptr_t
 _rtld_unw_getcontext_unsealed(uintptr_t ret, void **buf)
 {
+	if (!C18N_ENABLED) {
+		__attribute__((musttail))
+		return (_rtld_unw_getcontext_epilogue(ret, buf));
+	}
 	*buf = unwind_cursor();
 	return (ret);
 }
@@ -973,13 +988,6 @@ _rtld_longjmp(struct jmp_args ret, void *rcsp, void **buf)
 	return (unwind_stack(ret, rcsp, cheri_unseal(*buf, sealer_jmpbuf)));
 }
 
-/*
- * An assembly function that is called to complete the unwind when
- * compartmentalisation is disabled. The call must be a tail-call so that
- * registers are not clobbered.
- */
-struct jmp_args _rtld_unw_setcontext_epilogue(struct jmp_args, void *, void **);
-
 struct jmp_args
 _rtld_unw_setcontext(struct jmp_args ret, void *rcsp, void **buf)
 {