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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 11 Aug 2020 08:20:24 -0700
Message-ID: <CAHk-=wjzLmMRf=QG-n+1HnxWCx4KTQn9+OhVvUSJ=ZCQd6Y1WA@mail.gmail.com>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     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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[ I missed the beginning of this discussion, so maybe this was already
suggested ]

On Tue, Aug 11, 2020 at 6:54 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> >
> > E.g.
> >   openat(AT_FDCWD, "foo/bar//mnt/info", O_RDONLY | O_ALT);
>
> Proof of concept patch and test program below.

I don't think this works for the reasons Al says, but a slight
modification might.

IOW, if you do something more along the lines of

       fd = open(""foo/bar", O_PATH);
       metadatafd = openat(fd, "metadataname", O_ALT);

it might be workable.

So you couldn't do it with _one_ pathname, because that is always
fundamentally going to hit pathname lookup rules.

But if you start a new path lookup with new rules, that's fine.

This is what I think xattrs should always have done, because they are
broken garbage.

In fact, if we do it right, I think we could have "getxattr()" be 100%
equivalent to (modulo all the error handling that this doesn't do, of
course):

  ssize_t getxattr(const char *path, const char *name,
                        void *value, size_t size)
  {
     int fd, attrfd;

     fd = open(path, O_PATH);
     attrfd = openat(fd, name, O_ALT);
     close(fd);
     read(attrfd, value, size);
     close(attrfd);
  }

and you'd still use getxattr() and friends as a shorthand (and for
POSIX compatibility), but internally in the kernel we'd have a
interface around that "xattrs are just file handles" model.

               Linus