file metadata via fs API (was: [GIT PULL] Filesystem Information)

[Linus Torvalds]

[ 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 at 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