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

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Date:   Tue, 11 Aug 2020 14:35:33 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, 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 Mailing List <linux-kernel@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

On 8/11/2020 1:28 PM, Miklos Szeredi wrote:
> On Tue, Aug 11, 2020 at 6:17 PM Casey Schaufler <casey@...aufler-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?

The founders did lots of things that, in retrospect, weren't
such great ideas, but that we have to live with.

> 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.

Hee Hee. Back in the early days of UNIX (the 1970s) there was command
dsw(1) "delete from switches" because files with untypeible names where
unfortunately common. I would question the assertion that "applications
are not used to dumping binary data into file names", based on how
often I've wished we still had dsw(1).

>   And that
> means it's probably possible to find a fairly short combination that
> is never used in practice (probably containing the "/." sequence).

You'd think, but you'd be wrong. In the UNIX days we tried everything
from "..." to ".NO_HID." and there always arose a problem or two. Not
the least of which is that a "magic" pathname generated on an old system,
then mounted on a new system will never give you the results you want.

> Why couldn't we reserve such a combination now?
>
> I have no idea how to find such it, but other than that, I see no
> theoretical problem with extending the list of reserved filenames.

You need a sequence that is never used in any language, and
that has never been used as a magic shell sequence. If you want
a fun story to tell over beers, look up how using the "@" as the
erase character on a TTY33 lead to it being used in email addresses.

> Thanks,
> Miklos