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

[Linus Torvalds]

On Tue, Aug 11, 2020 at 9:17 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> This doesn't work so well for setxattr(), which we want to be atomic.

Well, it's not like the old interfaces could go away. But yes, doing

        metadatafd = openat(fd, "metadataname", O_ALT | O_CREAT | O_EXCL)

to create a new xattr (and then write to it) would not act like
setxattr(). Even if you do it as one atomic write, a reader would see
that zero-sized xattr between the O_CREAT and the write.

Of course, we could just hide zero-sized xattrs from the legacy
interfaces and avoid things like that, but another option is to say
that only the legacy interfaces give that particular atomicity
guarantee.

> 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.

Yeah, I really don't think it's a good idea to play with "//".

POSIX does allow special semantics for a pathname with "//" at the
*beginning*, but even that has been very questionable (and Linux has
never supported it).

                   Linus