[ python-Bugs-1177468 ] random.py/os.urandom robustness

[SourceForge.net]

Bugs item #1177468, was opened at 2005-04-06 01:03
Message generated for change (Comment added) made by jafo
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1177468&group_id=5470

Category: Python Library
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Fazal Majid (majid)
Assigned to: Nobody/Anonymous (nobody)
Summary: random.py/os.urandom robustness

Initial Comment:
Python 2.4.1 now uses os.urandom() to seed the random
number generator. This is mostly an improvement, but
can lead to subtle regression bugs.

os.urandom() will open /dev/urandom on demand, e.g.
when random.Random.seed() is called, and keep it alive
as os._urandomfd.

It is standard programming practice for a daemon
process to close file descriptors it has inherited from
its parent process, and if it closes the file
descriptor corresponding to os._urandomfd, the os
module is blissfully unaware and the next time
os.urandom() is called, it will try to read from a
closed file descriptor (or worse, a new one opened
since), with unpredictable results.

My recommendation would be to make os.urandom() open
/dev/urandom each time and not keep a persistent file
descripto. This will be slightly slower, but more
robust. I am not sure how I feel about a standard
library function steal a file descriptor slot forever,
specially when os.urandom() is probably going to be
called only once in the lifetime of a program, when the
random module is seeded.


----------------------------------------------------------------------

>Comment By: Sean Reifschneider (jafo)
Date: 2005-04-06 03:11

Message:
Logged In: YES 
user_id=81797

Just providing some feedback:

I'm able to reproduce this.  Importing random will cause
this file descriptor to be called.  Opening urandom on every
call could lead to unacceptable syscall overhead for some. 
Perhaps there should be a "urandomcleanup" method that
closes the file descriptor, and then random could get the
bytes from urandom(), and clean up after itself?

Personally, I only clean up the file descriptors I have
allocated when I fork a new process.  On the one hand I
agree with you about sucking up a fd in the standard
library, but on the other hand I'm thinking that you just
shouldn't be closing file descriptors for stuff you'll be
needing.  That's my two cents on this bug.

----------------------------------------------------------------------

Comment By: Fazal Majid (majid)
Date: 2005-04-06 01:06

Message:
Logged In: YES 
user_id=110477

There are many modules that have a dependency on random, for
instance os.tempnam(), and a program could well
inadvertently use it before closing file descriptors.

----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1177468&group_id=5470