Re: [RFC] readlink()-related oddities

David Howells
2015-11-20T09:59:23Z

Al Viro <viro@ZenIV.linux.org.uk> wrote:

> 3) normally, readlink(2) fails for non-symlinks. Moreover, according to
> POSIX it should do so (with -EINVAL). There is a pathological case when
> it succeeds for a directory, though. Namely, one of the kinds of AFS
> "mountpoints".

All AFS mountpoints are magic symlinks that are specially interpreted by the
client as far as I'm aware. I'm not sure why the designers didn't just select
a different file type for them, but they didn't.

Unfortunately, it means that iget has to read the contents of the symlinks :-/

> stat(2) reports those as directories, stepping into them leads to
> automounting a directory there (why do we have ->open() for them, BTW?).

I think I put that in to make sure the open() syscall returned EREMOTE rather
than another error if you tried to open it. It can probably be removed
because with the d_automount code you can't ever get there I think - unless
you can pass AT_NO_AUTOMOUNT to openat().

> How the hell is userland supposed to guess to call readlink(2) on those
> suckers to get the information of what'll get automounted there if we step
> upon them?

There's an AFS userspace command that could be used to query a mountpoint that
was going to use it. However, I suspect readlink() will now always trigger
the automount. This is one of the things OpenAFS uses pioctl() for - but
since I'm not allowed to add that to the kernel, I have to find some other way
of doing it.

> And could we please get rid of that kludge? David?

Sure.

David