1 files changed, 8 insertions, 3 deletions
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index c565404..b40513f 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -430,15 +430,20 @@ static PyObject *
 bytesio_readinto(bytesio *self, PyObject *buffer)
 {
     void *raw_buffer;
-    Py_ssize_t len;
+    Py_ssize_t len, n;
 
     CHECK_CLOSED(self);
 
     if (PyObject_AsWriteBuffer(buffer, &raw_buffer, &len) == -1)
         return NULL;
 
-    if (self->pos + len > self->string_size)
-        len = self->string_size - self->pos;
+    /* adjust invalid sizes */
+    n = self->string_size - self->pos;
+    if (len > n) {
+        len = n;
+        if (len < 0)
+            len = 0;
+    }
 
     memcpy(raw_buffer, self->buf + self->pos, len);
     assert(self->pos + len < PY_SSIZE_T_MAX);