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

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