fix(setjmp): restore caller rbx in sigsetjmp mask path

tests/test_setjmp.c

@@ -294,6 +294,30 @@ underscore_forms_test(void)
     check(r == 9, "_longjmp(env,9) -> _setjmp returned 9");
 }
 
+/*
+ * 7. The initial sigsetjmp(env, 1) return must preserve the caller's rbx.
+ * A canary is planted in rbx via inline assembly (declared clobbered, so
+ * the compiler holds nothing else there), sigsetjmp returns normally, and
+ * rbx is read back raw: the SysV AMD64 ABI requires the function to leave
+ * every callee-saved register intact on the ordinary return path, and the
+ * compiler assumes exactly that when allocating registers around the call,
+ * so nothing reloads rbx between the plant and the read.
+ */
+static __attribute__((noinline)) void
+rbx_callee_saved_test(void)
+{
+    sigjmp_buf env;
+    unsigned long after = 0;
+    int r;
+
+    __asm__ volatile("mov $0x12345678, %%rbx" ::: "rbx");
+    r = sigsetjmp(env, 1);
+    __asm__ volatile("mov %%rbx, %0" : "=r"(after));
+
+    check(r == 0, "sigsetjmp(env,1) initial return is 0 (rbx canary scenario)");
+    check(after == 0x12345678UL, "caller rbx preserved across sigsetjmp(env,1) initial return");
+}
+
 /*
  * Failure scenario (-f): longjmp(env, 0) must make setjmp return 1. Exits 0
  * only when the coercion behaved exactly as POSIX specifies.
@@ -334,6 +358,7 @@ main(int argc, char **argv)
     fp_state_test();
     sigmask_test();
     underscore_forms_test();
+    rbx_callee_saved_test();
 
     if (failures > 0)
     {