linux-kernel - Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

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Date:   Tue, 11 Aug 2020 16:47:41 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org, David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Karel Zak <kzak@...hat.com>, Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

On Tue, Aug 11, 2020 at 4:42 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Tue, Aug 11, 2020 at 04:36:32PM +0200, Miklos Szeredi wrote:
>
> > > >  - strip off trailing part after first instance of ///
> > > >  - perform path lookup as normal
> > > >  - resolve meta path after /// on result of normal lookup
> > >
> > > ... and interpolation of relative symlink body into the pathname does change
> > > behaviour now, *including* the cases when said symlink body does not contain
> > > that triple-X^Hslash garbage.  Wonderful...
> >
> > Can you please explain?
>
> Currently substituting the body of a relative symlink in place of its name
> results in equivalent pathname.

Except proc symlinks, that is.

>  With your patch that is not just no longer
> true, it's no longer true even when the symlink body does not contain that
> /// kludge - it can come in part from the symlink body and in part from the
> rest of pathname.  I.e. you can't even tell if substitution is an equivalent
> replacement by looking at the symlink body alone.

Yes, that's true not just for symlink bodies but any concatenation of
two path segments.

That's why it's enabled with RESOLVE_ALT.  I've said that I plan to
experiment with turning this on globally, but that doesn't mean it's
necessarily a good idea.  The posted patch contains nothing of that
sort.

Thanks,
Miklos