Symlinks already present

[Chris Angelico]

On Tue, Sep 1, 2020 at 10:57 PM Richard Damon <Richard at damon-family.org> wrote:
>
> On 8/31/20 6:05 PM, Chris Angelico wrote:
> > On Tue, Sep 1, 2020 at 5:08 AM Richard Damon <Richard at damon-family.org> wrote:
> >> The file descriptor could remember the path used to get to it. chroot
> >> shows that .. needs to be somewhat special, as it needs to go away for
> >> anyone that . is their current root.
> > But my point is that there might not actually *be* a valid path that
> > gets you to a file descriptor. It can't remember something that
> > doesn't exist. (And it's pretty impractical to do that even if it
> > does.)
>
> Remember, we are talking about a hypothetical OS that handles hardlinks
> to directories, and defines that .. will point to the parent used to
> come to it, NOT just having current *NIX allowing hardlinks to
> directories with no remediation of the issues cause.

Yes. I'm assuming that you can define things any way you like.

> One result of the definition, is that when you open a file/directory, if
> it might be a directory, the system WILL need to remember the path to it
> (so as to provide a value for ..) and that memory will provide a
> 'reference' for the directories so they can't go away (just like an
> unlinked file stays around will someone has it open). The normal way to
> get a file descriptor starts from a path, so the path had to exist in
> the first place, and since we are assuming that to get .., it keep a
> reference to that path, it can't truly go away.
>

But as I've pointed out, you don't always *have* a valid path. File
descriptors can be passed from process to process (most commonly by
being retained when you fork and not closed when you exec, but there
are other ways too, eg sockets), so even though starting with a path
is the most common, it isn't the only way, and you could easily have
an FD with no associated path, and then read it to find out what ".."
is.

Also, even if all that could be solved, I don't like the idea that
reading the same directory from two different sources leads to
different results. Is it really the same directory if reading it in
different ways gives different results?

ChrisA