diff --git a/tools/helpers/init-xenstore-domain.c b/tools/helpers/init-xenstore-domain.c
index 2be5831a57..6c0c85f032 100644
--- a/tools/helpers/init-xenstore-domain.c
+++ b/tools/helpers/init-xenstore-domain.c
@@ -108,7 +108,7 @@ static int build(xc_interface *xch)
 
     if ( flask )
     {
-        rv = xc_flask_context_to_sid(xch, flask, strlen(flask), &config.ssidref);
+        rv = xc_flask_context_to_sid(xch, flask, &config.ssidref);
         if ( rv )
         {
             fprintf(stderr, "xc_flask_context_to_sid failed\n");
diff --git a/tools/include/xenctrl.h b/tools/include/xenctrl.h
index f4316089e7..9f00d4a19d 100644
--- a/tools/include/xenctrl.h
+++ b/tools/include/xenctrl.h
@@ -2377,7 +2377,7 @@ long xc_sharing_used_frames(xc_interface *xch);
 /*** End sharing interface ***/
 
 int xc_flask_load(xc_interface *xc_handle, char *buf, uint32_t size);
-int xc_flask_context_to_sid(xc_interface *xc_handle, char *buf, uint32_t size, uint32_t *sid);
+int xc_flask_context_to_sid(xc_interface *xc_handle, char *buf, uint32_t *sid);
 int xc_flask_sid_to_context(xc_interface *xc_handle, int sid, char *buf, uint32_t size);
 int xc_flask_getenforce(xc_interface *xc_handle);
 int xc_flask_setenforce(xc_interface *xc_handle, int mode);
diff --git a/tools/libs/ctrl/xc_flask.c b/tools/libs/ctrl/xc_flask.c
index f5389dbd96..f275093735 100644
--- a/tools/libs/ctrl/xc_flask.c
+++ b/tools/libs/ctrl/xc_flask.c
@@ -83,10 +83,11 @@ int xc_flask_load(xc_interface *xch, char *buf, uint32_t size)
     return err;
 }
 
-int xc_flask_context_to_sid(xc_interface *xch, char *buf, uint32_t size, uint32_t *sid)
+int xc_flask_context_to_sid(xc_interface *xch, char *buf, uint32_t *sid)
 {
     int err;
     struct xen_flask_op op = {};
+    size_t size = strlen(buf) + 1;
     DECLARE_HYPERCALL_BOUNCE(buf, size, XC_HYPERCALL_BUFFER_BOUNCE_IN);
 
     if ( xc_hypercall_bounce_pre(xch, buf) )
@@ -249,7 +250,7 @@ static int xc_flask_add(xc_interface *xch, uint32_t ocon, uint64_t low, uint64_t
     int err;
     struct xen_flask_op op = {};
 
-    err = xc_flask_context_to_sid(xch, scontext, strlen(scontext), &sid);
+    err = xc_flask_context_to_sid(xch, scontext, &sid);
     if ( err )
         return err;
 
@@ -325,10 +326,10 @@ int xc_flask_access(xc_interface *xch, const char *scon, const char *tcon,
     struct xen_flask_op op = {};
     int err;
 
-    err = xc_flask_context_to_sid(xch, (char*)scon, strlen(scon), &op.u.access.ssid);
+    err = xc_flask_context_to_sid(xch, (char*)scon, &op.u.access.ssid);
     if ( err )
         return err;
-    err = xc_flask_context_to_sid(xch, (char*)tcon, strlen(tcon), &op.u.access.tsid);
+    err = xc_flask_context_to_sid(xch, (char*)tcon, &op.u.access.tsid);
     if ( err )
         return err;
 
diff --git a/tools/libs/light/libxl_flask.c b/tools/libs/light/libxl_flask.c
index 38347a31a3..8d9c92bfdc 100644
--- a/tools/libs/light/libxl_flask.c
+++ b/tools/libs/light/libxl_flask.c
@@ -21,7 +21,10 @@ int libxl_flask_context_to_sid(libxl_ctx *ctx, char *buf, size_t len,
 {
     int rc;
 
-    rc = xc_flask_context_to_sid(ctx->xch, buf, len, ssidref);
+    if (len != strlen(buf))
+        return ERROR_INVAL;
+
+    rc = xc_flask_context_to_sid(ctx->xch, buf, ssidref);
 
     return rc;
 }
diff --git a/tools/python/xen/lowlevel/xc/xc.c b/tools/python/xen/lowlevel/xc/xc.c
index a94f01d40b..7a4bf54597 100644
--- a/tools/python/xen/lowlevel/xc/xc.c
+++ b/tools/python/xen/lowlevel/xc/xc.c
@@ -1754,7 +1754,7 @@ static PyObject *pyflask_context_to_sid(PyObject *self, PyObject *args,
         return PyErr_SetFromErrno(xc_error_obj);
     }
 
-    ret = xc_flask_context_to_sid(xc_handle, ctx, strlen(ctx), &sid);
+    ret = xc_flask_context_to_sid(xc_handle, ctx, &sid);
 
     xc_interface_close(xc_handle);