Re: symlink_prefix

[Alexander Viro]

On Sun, 3 Jun 2001 [e-mail] wrote:

> This evening I needed to work on a filesystem of a non-Linux OS,
> full of absolute symlinks. After mounting the fs on /mnt, each
> symlink pointing to /foo/bar in that filesystem should be
> regarded as pointing to /mnt/foo/bar.
> 
> Since doing ls -ld on every component of every pathname was
> far too slow, I made a small kernel wart, where a mount option
> -o symlink_prefix=/pathname would cause /pathname to be prepended
> in front of every absolute symlink in the given filesystem
> (when the symlink is followed). That works satisfactorily.
> 
> Remain the questions:
> (i) is there already a mechanism that would achieve this?
> (ii) if not, do we want something like this in the kernel?
> 
> There is already a vaguely similar (and much uglier) wart,
> namely that of "altroot". It is really ugly - requires a path
> set at kernel compile time. And the scope is different.
> Instead of all processes and a single filesystem and symlinks only,
> altroot affects a single process and all filesystems and all paths.
> 
> I do not immediately see a common generalization of these two.

altroot should be buried, not generalized. It was a mistake and
we will be better off forgetting about that nightmare instead of
trying to design something around it.

Absolute symlinks... Dunno. _If_ we want that at all, we probably
want it on per-mountpoint basis. However, that opens a door to
_really_ ugly feature requests. E.g. "if symlink starts with
/foo - replace it with /mnt/bar, but if it starts with /foo/baz -
replace with /mnt/splat instead".

I can see how to implement per-mountpoint variant. However, I'm
less than enthusiastic about the API side of that and about the
ugliness it will lead to. It smells like a wrong approach. And
no, I don't see a good one right now.

As for the API... How would you pass that option? Yet another
mount(2) argument?