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From:   Jann Horn <jannh@google.com>
Date:   Tue, 11 Aug 2020 22:36:58 +0200
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Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Casey Schaufler <casey@schaufler-ca.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Karel Zak <kzak@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Christian Brauner <christian@brauner.io>,
        Lennart Poettering <lennart@poettering.net>,
        Linux API <linux-api@vger.kernel.org>,
        Ian Kent <raven@themaw.net>,
        LSM <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Aug 11, 2020 at 10:29 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> On Tue, Aug 11, 2020 at 6:17 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> > Since a////////b has known meaning, and lots of applications
> > play loose with '/', its really dangerous to treat the string as
> > special. We only get away with '.' and '..' because their behavior
> > was defined before many of y'all were born.
>
> So the founding fathers have set things in stone and now we can't
> change it.   Right?
>
> Well that's how it looks... but let's think a little; we have '/' and
> '\0' that can't be used in filenames.  Also '.' and '..' are
> prohibited names. It's not a trivial limitation, so applications are
> probably not used to dumping binary data into file names.  And that
> means it's probably possible to find a fairly short combination that
> is never used in practice (probably containing the "/." sequence).
> Why couldn't we reserve such a combination now?

This isn't just about finding a string that "is never used in
practice". There is userspace software that performs security checks
based on the precise semantics that paths have nowadays, and those
security checks will sometimes happily let you use arbitrary binary
garbage in path components as long as there's no '\0' or '/' in there
and the name isn't "." or "..", because that's just how paths work on
Linux.

If you change the semantics of path strings, you'd have to be
confident that the new semantics fit nicely with all the path
validation routines that exist scattered across userspace, and don't
expose new interfaces through file server software and setuid binaries
and so on.

I really don't like this idea.