For optimum security, a random salt should be chosen for PBKDF2.
I came across a scenario where I produce a hash of a random file that serves as the "password". Due to the nature of files, this has similar entropy issues as a "normal" password.
What would happen now if I decided to take the same (low-entropy) hash as the salt, too? Is there a known attack on this? Of course, in this scenario the salt would be kept secret. But still, since it is not entirely random, does this compromise the overall security, and if so, how badly? I know that for example "key as IV" severely compromises certain ciphers, so I'm afraid it could do harm in this situation, too.