Re: [PATCH] do_open(): Fix O_DIRECTORY | O_CREAT behavior

[Josh Triplett]

On Mon, Mar 20, 2023 at 10:14:29AM -0700, Linus Torvalds wrote:
> On Mon, Mar 20, 2023 at 4:52 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> >
> > So before we continue down that road should we maybe treat this as a
> > chance to fix the old bug? Because this behavior of returning -ENOTDIR
> > has existed ever since v5.7 now. Since that time we had three LTS
> > releases all returning ENOTDIR even if the file was created.
> 
> Ack.
> 
> I think considering that the return value has been broken for so long,
> I think we can pretty much assume that there are no actual users of
> it, and we might as well clean up the semantics properly.

If there are no users of this and we can clean up the semantics, is
there a strong reason *not* to make `O_DIRECTORY | O_CREATE` actually
create a directory and atomically return a file descriptor for that
directory? That seems like genuinely useful behavior that we don't
currently have a syscall for. I didn't see any suggestion in the thread
for reasons why we can't or shouldn't do that.

Would that break some existing software? It doesn't *sound* like it
would.

As far as I can tell, that *also* wouldn't cause a problem with
O_TMPFILE, because older kernels will still fail as desired, and we
wouldn't change the behavior of O_TMPFILE on new kernels (it'd still
create a temporary file, not a directory).

- Josh Triplett