Re: [RFC] readlink()-related oddities

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Nov 19, 2015 at 06:13:53PM -0800, Linus Torvalds wrote:

> > 3) normally, readlink(2) fails for non-symlinks.  Moreover, according to
> > POSIX it should do so (with -EINVAL).
> 
> I don't think POSIX is necessarily relevant here.
> 
> We have had magic file behavior outside the scope of POSIX before, and
> we will have it in the future. It makes perfect sense to use
> readlink() for management tools for automounting, even if the normal
> oepration is to treat the thing as a directory.
> 
> Not everything is within the domain of POSIX.

How would those tools know that this particular pathname _is_ a magical
symlink?  Sure, if you see a symlink with body that starts with % or #,
you could figure out that it's not a regular one and go parse the body,
but for stat(2) it looks like a directory.  Do those tools call readlink()
on every directory they spot on AFS volume?  David?

And what's the story with magical ->open() for those?  How could one get
to ->open() on those sucker and avoid triggering the automount instead?