Re: Publically writable directories

[jim () california sandia gov (Jim Hutchins)]

Bill Pemberton wrote:
> Jim Hutchins writes:
> > After thinking about this and watching some of the replies, I believe
> > the absolute answer is no, you cannot avoid a race condition on all
> > platforms.  open(2) will follow a symbolic link on the platforms I
> > tried, with or without O_EXCL.  Depending on what you are trying to
> > do, a couple possible solutions are:
>
> Which platforms were that?  For the elm bug I tested the code on a bunch of
> platforms and it did not follow the symlink.  The platforms I personally
> tested it on included AIX 3, AIX 4, Solaris 2, SunOS 4, IRIX 4, IRIX 5.  The
> other elm folks have tried on many other operating systems.
>
> I've assumed that there were some systems out there that didn't behave
> correctly, but it appears to be rare.

I should have tested more then two machines for a statistical sample,
especially two from different vendors.  The machine on my desk (IRIX
4.0.5F) and another I had a window open to (IRIX 4.0.1) both follow
symlinks on open with a O_EXCL.  I just tested a few other systems
(IRIX 5.3, SunOS 4.1.2 and 4.1.3_U1, Ultrix 4.1, 4.2 and 4.3A, and DEC
OSF/1 2.1 and 3.2) and they all failed with a "file exists" error.

So for most platforms, it looks like doing an open with O_EXCL is safe,
though I believe my original statement is still true - "the absolute
answer is no, you cannot avoid a race condition on ALL platforms."

Jim Hutchins
Sandia National Labs, California

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