diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 56f29c35b5..e8a5aede95 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -20,6 +20,11 @@
 
 #define THREAD __thread
 
+/* This is the size of the host kernel's sigset_t, needed where we make
+ * direct system calls that take a sigset_t pointer and a size.
+ */
+#define SIGSET_T_SIZE (_NSIG / 8)
+
 /* This struct is used to hold certain information about the image.
  * Basically, it replicates in user space what would be certain
  * task_struct fields in the kernel
diff --git a/linux-user/signal.c b/linux-user/signal.c
index e2d55fff97..9d980456ec 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -636,8 +636,16 @@ static void host_signal_handler(int host_signum, siginfo_t *info,
      * code in case the guest code provokes one in the window between
      * now and it getting out to the main loop. Signals will be
      * unblocked again in process_pending_signals().
+     *
+     * WARNING: we cannot use sigfillset() here because the uc_sigmask
+     * field is a kernel sigset_t, which is much smaller than the
+     * libc sigset_t which sigfillset() operates on. Using sigfillset()
+     * would write 0xff bytes off the end of the structure and trash
+     * data on the struct.
+     * We can't use sizeof(uc->uc_sigmask) either, because the libc
+     * headers define the struct field with the wrong (too large) type.
      */
-    sigfillset(&uc->uc_sigmask);
+    memset(&uc->uc_sigmask, 0xff, SIGSET_T_SIZE);
     sigdelset(&uc->uc_sigmask, SIGSEGV);
     sigdelset(&uc->uc_sigmask, SIGBUS);
 
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 3dfaea9c8e..5166ff9b62 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -123,11 +123,6 @@ int __clone2(int (*fn)(void *), void *child_stack_base,
 #define	VFAT_IOCTL_READDIR_BOTH		_IOR('r', 1, struct linux_dirent [2])
 #define	VFAT_IOCTL_READDIR_SHORT	_IOR('r', 2, struct linux_dirent [2])
 
-/* This is the size of the host kernel's sigset_t, needed where we make
- * direct system calls that take a sigset_t pointer and a size.
- */
-#define SIGSET_T_SIZE (_NSIG / 8)
-
 #undef _syscall0
 #undef _syscall1
 #undef _syscall2