Messages in this thread | | | Date | Sat, 15 Jan 2005 18:44:46 -0800 | From | Matt Mackall <> | Subject | Re: short read from /dev/urandom |
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On Thu, Jan 13, 2005 at 08:54:54PM -0800, Ulrich Drepper wrote: > The /dev/urandom device is advertised as always returning the requested > number of bytes. Yet, it fails to do this under some situations. > Compile this
Here's what random.c says:
* The two other interfaces are two character devices /dev/random and * /dev/urandom. /dev/random is suitable for use when very high * quality randomness is desired (for example, for key generation or * one-time pads), as it will only return a maximum of the number of * bits of randomness (as estimated by the random number generator) * contained in the entropy pool. * * The /dev/urandom device does not have this limit, and will return * as many bytes as are requested. As more and more random bytes are * requested without giving time for the entropy pool to recharge, * this will result in random numbers that are merely cryptographically * strong. For many applications, however, this is acceptable.
_Neither_ case mentions signals and the "and will return as many bytes as requested" is clearly just a restatement of "does not have this limit". Whoever copied this comment to the manpage was a bit sloppy and dropped the first clause rather than the second:
When read, /dev/urandom device will return as many bytes as are requested. As a result, if there is not sufficient entropy in the entropy pool...
Meanwhile, read(2) says:
It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal.
So anyone doing a read() can expect a short read regardless of the fd and is quite clear that reads can be interrupted by signals. "It is not an error". Ever.
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