vm: fix invalid memory access on toplevel function calls

When a function is invoked directly, e.g. through uc_vm_invoke(), the call
stack is empty, so avoid accessing the first call frame unless we actually
need to, which is only the case if the function is invoked with spread args
which can only happen with calls from manged code.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 3 insertions, 1 deletions

diff --git a/vm.c b/vm.c
index 9997580..a5c4b8c 100644
--- a/vm.c
+++ b/vm.c
@@ -447,7 +447,7 @@ static bool
 uc_vm_call_function(uc_vm *vm, uc_value_t *ctx, uc_value_t *fno, bool mcall, size_t argspec)
 {
 	size_t i, j, stackoff, nargs = argspec & 0xffff, nspreads = argspec >> 16;
-	uc_callframe *frame = uc_vm_current_frame(vm);
+	uc_callframe *frame = NULL;
 	uc_value_t *ellip, *arg;
 	uc_function_t *function;
 	uc_closure_t *closure;
@@ -465,6 +465,8 @@ uc_vm_call_function(uc_vm *vm, uc_value_t *ctx, uc_value_t *fno, bool mcall, siz
 
 	/* argument list contains spread operations, we need to reshuffle the stack */
 	if (nspreads > 0) {
+		frame = uc_vm_current_frame(vm);
+
 		/* create temporary array */
 		ellip = ucv_array_new_length(vm, nargs);