> * User has maliciously modified file and set mtime back. Since it's MD5 (broken), the malicious user can make the checksum match too. checksumming doesn't hel

> * User has maliciously modified file and set mtime back. Since it's MD5 (broken), the malicious user can make the checksum match too. checksumming doesn't help. No, md5 is not broken like that (yet). There is no nkown second-preimage attack against md5; the practical collision vulns only affect cases where an attack controls the file content both before and after update.