CSC369 Exam Cheat Sheet Fall 2013.pdf

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University of Toronto St. George
Computer Science
Angela Demke Brown

Conflicting operations if both access the DeadlocksTransactionsNew Technology File System NTFSFiles and Dirs File System Layoutssame data item and at least one is a write if a Dfn permanent blocking of a set of processesDesigners had the following goalsDirs are files so inodes also describe where serial schedule can be gotten by moving that 1 compete for system rsrcs or 2 1 Eliminate fixedsize short namesthe blocks for directories are placed on the around nonconflicting operations then original communicate with each other2 Implement thorough permission schemedisk Dir entries map filenames to inodesschedule is conflictserializable Conditions Mutex one at a time Hold3 Provide good performanceUsing inodes to open oneEnsuring serializability Two Phase Locking Wait proc may hold allocated resources while 4 Support large files1to open one user Master Block to findeach transaction has a growing phase and waiting No Preemption Resources cannot 5 Provide extra functionality compression inode forand read inode into memory shrinking phase growingonly obtain locks be removed from process holding it Circular encryption types inode allows us to find data block for dir2 shrinkingonly release locks Does not Wait Closed chain of processes exist whereEach volume partition is a linear sequence Readlook entry for one This entry gives guarantee deadlockfree Fix prevent hw by each process holds at least one resource of blocks usually 4 KBlocation of inode for one 3 Read inode for aborting and retrying transactions Timestamp needed by next process in chain ImpliesEach volume has Master File Table MTFone into memoryThe inodes says where protocols all transactions get unique holdwait and is a sequence of events butSequence of 1 KB recordsfirst data block is on disk 7 Read that block timestamp before it execs Each data item gets hw is a policy decision One or more records per file or directoryinto memory to access data in filetwo timestamps writets and readts TSTogether these four conditions are similar to inode but more flexibleFiles and Dirs File Buffer CacheOrdering Reads if trans has earlier ts than necessary and sufficient for deadlock Each MFT record is a sequence of variable Cache file blocks in memory to capture writets on data than trans needs to abort We are only interested in reusable resources length pairs locality Caching is systemtransaction restart with new ts Write if trans Deadlock Prevention make sure one of theLong attributes can be stored externally wide usedshared by all processes has earlier ts than readts writets on data following doesnt occur Break mutex and a pointer kept in MFT recordWrite some progsassume that data then the value produced by the write should required for correctness Break hw youNTFS fries to allocate files in runs of makes it from the buffer into the disk As a have been read alreadySome transactions can end up waiting for a long timecausing consecutive blocksresult writing is slow even with caching Use may starve so abort and restart starvation if a process locks some resources Securitywritebehind maintain queue of repeatedlyand waits for others to be released Another 4 Requirements of Securityuncommitted writes with NVRAM to make Files and Directoriesalternative is to try to lock everything in order 1Confidentiality prevent unauthorized reliable OR log structured FS to write at end There are two tables to keep track of open and if you fail to acquire a lock for a resource release of info 2Integrity prevent of previous writefiles a systemwide table and perprocess release all previous locks and try again later unauthorized modification of info 3Read use readahead via predicting next table which points to the systemwide table Break no preemption forcibly remove a Availability ensuring access to legitimate block take advantage of locality and Supported File Operationsresource and assign it to another You might users 4Authenticity verifying the identity of a compliment ondisk cache 1creating 2writing 3reading 4deleting 5need to save the state of the process you are user CryptographyTechniques for Tradeoff vs VM competes with VM need repositioning within a file 6 truncatingerase grabbing it from and roll it back to an earlier communicating in the presence of adversariesreplacement algorithms LRU usually used parts keep attributesstate This is impossible for consumable Types of ThreatsLimited SizeFiles Characteristics metadataresources Break circular wait Assign a 1InterceptionEavesdropping Attack on Disk Scheduling1protection 2creator 3owner 4creationlast linear order to resource types and require that confidentiality 2Modification Attack on FCFSReasonable when load is lowaccesslast change a process holding a resource of one type can integrity Special case is Destruction Attack SSTF Shortest seek time firstMinimizes time 5current sizeonly request resources that followit in the on availability 3Theft of service Attacker arm movement maximizes request rate File Access Methodsordering Hard to come up with order installs daemon Attack on availability 4favours middle blks can lead to starvationSequential Access bytebybyte in orderAvoidance Strategies 1 Do not start a proc Fabrication Attack on authenticitySCAN elevatorRead in 1 direction then revDirect Access random access given if its max resource req together with the max VulnerabilitiesCSCAN typewriter SCAN but 1 directionblockbyte numberneeds of all procs already running aretotal Physical access Humans OS NetworksLOOKCLOOKLike SCANCSCAN but only Directory Benefits For users file sys resources Pessimistic assume all procs Login spoofinggoes as far as last request in each direction organization For Sys convenient naming will need all their resources at same time Malicious sftwr Trap doorsprogram contains as opposed to hitting beginning and end of interface logical file placement For all store Requires procs to declare max resource needs secret entry point that allows attacker to diskfile infoupfront 2 Do not grant indv res request if it bypass security logic bombsdestructive codeIn GeneralDirectory Implementationmight lead to deadlock Safe State there is at in legitimate program triggered by some event Unless there are request queues disc sched A Directory Entry is a list of names and least one seq of proc executions that does nottrojansapparently useful program that tricks has low impact Important for server so so for associated metadata lead to deadlock even if every proc requests users into running it May contain logic bomb PC Modern disks do their own schedulingTreestructured Acyclical graph if hard links their max alloc immediatelyUnsafe Stateor vi
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