The uid field of the cred struct is normally the second field, followed
by the gid field. The first field is of type atomic_t, which has the
size of an int. Since the size of an int is usually 4 bytes, the uid is
normally located at an offset of 4 bytes from the start of the cred
struct, and not 8. Since the uid also is int-sized, the code set
test_uid to the gid, making the exploit fail for cases where uid != gid.
Gustaf Blomqvist