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From r..@covalent.net
Subject Re: Deja vu
Date Sat, 06 Jan 2001 00:41:20 GMT

> > Ah, I see the problem.  It isn't apr_home_dir that would cause the
> > problem.  The problem is that under the covers apr_home_dir calls
> > getpwnam, which access the password database using static memory in the C
> > Run-Time.  We can put locks around getpwnam in apr_home_dir, and this will
> > ensure that multiple calls to apr_home_dir won't affect each other.
> > 
> > The problem comes in when some other C Run-Time function that access the
> > password database is called.  These potentially uses the same static
> > memory in the C Run-Time that getpwname uses, so we might overwrite the
> > data that getpwnam wants.
> > 
> > The locks that we put around getpwnam in apr_home_dir won't protect us if
> > some other part of Apache calls into the C Run-Time's password db
> > accessors without locking the mutex.
> 
> ??? But surely they shouldn't do that, coz they should call APRs
> routines for doing it?

But, the only APR function we have that deals with the password database
in apr_home_directory.  We decided when we first hit this issue, that
we wouldn't create all the other functions until we needed them.  In the
long term though, yes you are correct that if the only access to the
password database is through APR, we could solve all the problems.  I have
a hard time believing we can rely on programmers to accept that though.

> Actually, after contemplation, I thought the deal was that although you
> could lock getpwnam (and anything else that might obviously collide) you
> may still get bitten by something less obvious that collides with
> something getpwnam calls. I don't really buy this argument, because the
> obvious counter is to use a _universal_ mutex for _all_ non-thread-safe
> stuff, then they can interact as much as they want, coz only one will be
> running in practice. Stuff that uses this without APR's assistance in
> Apache is just plain broken, and not to be worried about. IMO.

You are correct, this is just plain broken, but at what point are you sure
that you have all of the functions covered in APR?  Unless you really
investigate the C Run-time to find ALL of the interactions, there is a
good chance you will miss one or two.

Ryan

_______________________________________________________________________________
Ryan Bloom                        	rbb@apache.org
406 29th St.
San Francisco, CA 94131
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