CS202: HW 11: File systems, fault-tolerance, NFS, security

Is it fundamentally necessary to store on disk information about the unallocated disk sectors (the bitmap, etc.)? Explain why or why not.

Consider a UNIX-like file system with multi-level indexing. For more reliability, the inode array is replicated on the disk in two different places. The intent is that if one or a group of sectors that are storing either replica of the array becomes corrupted, the system can always recover from the other replica. Discuss the effect of having this replicated data structure on performance.

1. List the reads and writes that must be executed to accomplish this task (and explain how each disk address is determined.)
2. Write down the writes in the order they should occur.
3. List the actions that the FSCK program must take to ensure the disk is brought to a correct state on recovery, and argue that these actions guarantee file system consistency.
4. Suppose we want to ensure reliable updates using logging (as in a journaling file system). List the series of writes (to the disk and log) in the order they should occur. Also describe what actions (if any) must be taken on recovery after a crash.

Consider a copy-on-write filesystem. Alice executes rm /home/alice/lab.c, where lab.c has 2 data blocks. Assume all directory entries are in direct blocks. List the operations performed by the filesystem when deleting lab.c, and specify any ordering constraints that must be obeyed between operations. Additionally, state any assumptions you make about how directory inodes are stored.

Consider the redo-undo logging protocol from lecture. Why can’t we eliminate the undo pass, and just have a redo pass in which we redo only sub-operations from committed transactions?

The NFS authors had a goal of transparency. They wanted applications to be unable to distinguish whether a file system was (a) a remote file system served from an NFS server; or (b) a typical, local Unix file system. They did not succeed. (In fact, their goal was impossible.) State precisely one way in which application code can experience different behavior when interacting with a remote NFS file system versus a local Unix file system. Your answer should be in terms of what application code sees (rather than in terms of what a global observer sees). Furthermore, please give the reason why it was impossible to provide transparency for the use case you stated.

For the statements below, please state whether they are true or false. Justify each answer.

1. "If a server has a buffer overflow vulnerability, that means the server definitely has a bug."
2. "Buffer overflow vulnerabilities can be ruled out by making the stack non-executable."
3. "Buffer overflow vulnerabilities can be ruled out by making program text read-only."
4. "Buffer overflow vulnerabilities can be ruled out with the W XOR X security policy."
5. "Buffer overflow vulnerabilities can be eliminated with ASLR (address space layout randomization)."
6. "Buffer overflow vulnerabilities are not possible on a 64-bit architecture."
7. "If a buffer overflow vulnerability is exploited, this implies that the attacker has changed %cr3."