Re: Why side-effects on open(2) are evil. (was Re: [RFD w/info-PATCH]device arguments from lookup)

Daniel Phillips
2001-05-27T20:44:11Z

On Sunday 27 May 2001 15:32, Edgar Toernig wrote:
> Daniel Phillips wrote:
> > It won't, the open for "." is handled in the VFS, not the
> > filesystem - it will open the directory. (Without needing to be
> > told it's a directory via O_DIRECTORY.) If you do open("magicdev")
> > you'll get the device, because that's handled by magicdevfs.
>
> You really mean that "magicdev" is a directory and:
>
> open("magicdev/.", O_RDONLY);
> open("magicdev", O_RDONLY);
>
> would both succeed but open different objects?

Yes, and:

open("magicdev/.", O_RDONLY | O_DIRECTORY);
open("magicdev", O_RDONLY | O_DIRECTORY);

will both succeed and open the same object.

> > I'm not claiming there isn't breakage somewhere,
>
> you break UNIX fundamentals. But I'm quite relieved now because I'm
> pretty sure that something like that will never go into the kernel.

OK, I'll take that as "I couldn't find a piece of code that breaks, so
it's on to the legal issues".

SUS doesn't seem to have a lot to say about this. The nearest thing to
a ruling I found was "The special filename dot refers to the directory
specified by its predecessor". Which is not the same thing as:

open("foo", O_RDONLY) == open ("foo/.", O_RDONLY)

I don't know about POSIX (I don't have it: a pox on standards
organizations that don't make their standards freely available) but SUS
doesn't seem to forbid this.

--
Daniel