[HEADSUP] Disallowing read() of a directory fd

[Alan Somers]

On Thu, May 14, 2020 at 2:17 PM Julian H. Stacey <jhs at berklix.com> wrote:

> Kyle Evans wrote:
> > Hi,
> >
> > This is a heads up, given that I'm completely flipping our historical
> > behavior- I intend to commit this review in a couple days' time
> > without substantial objection: https://reviews.freebsd.org/D24596
> >
> > With this, FreeBSD 13 will not allow read() of a directory fd, which
> > could have previously returned some data from the underlying
> > filesystem in no particular standardized format.
> >
> > This is a still-standards-compliant switch from one
> > implementation-defined behavior to another that's already been adopted
> > in various other popular kernels, to include OpenBSD, MacOS, and
> > Linux.
> >
> > Worth noting is that there's not really one largely-compelling reasons
> > to switch this after so many years (unless you find yourself that
> > irate when you accidentally `cat` a directory), but there are some
> > benefits which are briefly discussed in the commentary around the
> > review along with the history of the current behavior.
> >
> > This change also simplifies filesystem implementations to some extent.
> >
> > Thanks,
> >
> > Kyle Evans
>
> There is ZERO need for a spurious change at 2 days notice after 42+ years !
>
> "cat ." as been supported since Unix V6 1978 or earlier,
> no problem, even occasionaly useful.
>

Really?  When is that occasionally useful?  I've never seen anything useful
come out of reading a directory.