On Thu, May 22, 2003 at 12:21:10PM -0400, Derek Elkins wrote:
Why do you need such a unique name, using the open call you can always choose another if it already exists. One way or the other you still need to atomically check for security reasons, no matter how unique your name is your code shouldn't rely on the file not being created between checking and creation, uniqueness means little for a malicious attack. Simply using a random number generator would seem sufficient, though I'd probably do, at least, progname++num, though more so that the user can see what's related to what (if files get left around) than on the off chance of 4 billion numerically named files.
like someone else mentioned, NFS (and probably some other filesystems) have wierd semantics where O_CREAT | O_EXCL don't work properly always. on such broken filesystems there is not much you can do, but to make the system as robust as possible one should use everything at their disposal. I should also mention that I didn't just make up the previous formula for 'robust' temporary files. they are used in various applications and that is 'best common practice'. see the Maildir format for another example. as for 32bit random numbers being sufficient, I have one technical argument against it and one anecdote. The technical argument is called 'the birthday attack', a web search will provide lots of info on it. The upshot is the 32 bits is not nearly as secure as you think because probability does not work the way our intuition says. The anecdote involved a certain distributed operating system which when booted, would wait a random amount of time to connect to the server since there would be many machines, and all of them connecting at once would wedge the server. all seemed well until their first power outage, the power came back up and 3 minutes later the whole system came tumbling down much to their surprise. their random number generator was seeded from their clock, since the power came on for all systems at the same time, all the random number generators were seeded with the same value and hence the first thing that came out of them was the same for every system. The moral, random numbers arn't always random :) For the quick and dirty behind the birthday attack ask yourself this: how many people do you need before two of them most likely (> 50% chance) share a birthday? if you said anything greater than about 20 then that is too high. basically the number of random samples needed before two collide is much smaller than one thinks, it grows as the square root so a bigger space (like 32bits) doesn't help as much as one might think. -- --------------------------------------------------------------------------- John Meacham - California Institute of Technology, Alum. - john@foo.net ---------------------------------------------------------------------------