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

src/setjmp/x86_64/sigsetjmp.s

@@ -28,6 +28,12 @@
  *
  * The mask slot is written through &env[11]; on success the flag is set and
  * 0 is returned in eax, exactly like plain setjmp.
+ *
+ * rbx carries the env base across the syscall (rdi/rsi/rdx/r10 hold the
+ * syscall arguments, rax the number, rcx/r11 are clobbered by the kernel).
+ * rbx is callee-saved: its original value was stored in env[0] above, so it
+ * is reloaded from there BEFORE the ret, leaving every callee-saved register
+ * intact on the ordinary return path as the SysV AMD64 ABI requires.
  */
 
 .global __sigsetjmp
@@ -51,7 +57,7 @@ sigsetjmp:
 	fnstcw 72(%rdi)
 	test %esi,%esi
 	jz 1f
-	mov %rdi,%rbx        /* env base (rbx is restored from the buffer later) */
+	mov %rdi,%rbx        /* env base (scratch; caller rbx is in env[0]) */
 	mov $14,%eax         /* SYS_rt_sigprocmask */
 	xor %edi,%edi        /* how = SIG_BLOCK(0) */
 	xor %esi,%esi        /* set = NULL (query only) */
@@ -59,6 +65,7 @@ sigsetjmp:
 	mov $8,%r10d         /* sigsetsize (x86_64 sigset_t = one word) */
 	syscall
 	movq $1,80(%rbx)     /* mask saved: set the flag */
+	mov 0(%rbx),%rbx     /* restore the caller's rbx before returning */
 1:
 	xor %eax,%eax
 	ret

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)
     {