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The condition contradicts the exception text:
 if (offset >= st.st_size) {
 PyErr_SetString(PyExc_ValueError,
 "mmap offset is greater than file size");
 return NULL;
 }
The condition should be changed to (offset > st.st_size), similar to the later condition which is correct:
 } else if (offset + (size_t)map_size > st.st_size) {
 PyErr_SetString(PyExc_ValueError,
 "mmap length is greater than file size");
 return NULL;
 }
The patch is attached.