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SubjectRe: file metadata via fs API (was: [GIT PULL] Filesystem Information)
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> On Aug 11, 2020, at 8:20 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> [ 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.
>
>

This is a lot like a less nutty version of NTFS streams, whereas the /// idea is kind of like an extra-nutty version of NTFS streams.

I am personally not a fan of the in-band signaling implications of overloading /. For example, there is plenty of code out there that thinks that (a + “/“ + b) concatenates paths. With /// overloaded, this stops being true.
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