Re: [PATCH 01/14] VFS: Add additional RESOLVE_* flags [ver #18]

[Linus Torvalds]

On Thu, Mar 12, 2020 at 2:08 AM Stefan Metzmacher <metze@xxxxxxxxx> wrote:
>
> The whole discussion was triggered by the introduction of a completely
> new fsinfo() call:
>
> Would you propose to have 'at_flags' and 'resolve_flags' passed in here?

Yes, I think that would be the way to go.

> > If we need linkat2() and friends, so be it. Do we?
>
> Yes, I'm going to propose something like this, as it would make the life
> much easier for Samba to have the new features available on all path
> based syscalls.

Will samba actually use them? I think we've had extensions before that
weren't worth the non-portability pain?

But yes, if we have a major package like samba use it, then by all
means let's add linkat2(). How many things are we talking about? We
have a number of system calls that do *not* take flags, but do do
pathname walking. I'm thinking things like "mkdirat()"?)

> In addition I'll propose to have a way to specify the source of
> removeat and unlinkat also by fd in addition to the the source parent fd
> and relative path, the reason are also to detect races of path
> recycling.

Would that be basically just an AT_EMPTY_PATH kind of thing? IOW,
you'd be able to remove a file by doing

   fd = open(path.., O_PATH);
   unlinkat(fd, "", AT_EMPTY_PATH);

Hmm. We have _not_ allowed filesystem changes without that last
component lookup. Of course, with our dentry model, we *can* do it,
but this smells fairly fundamental to me.

It might avoid some of the extra system calls (ie you could use
openat2() to do the path walking part, and then
unlinkat(AT_EMPTY_PATH) to remove it, and have a "fstat()" etc in
between the verify that it's the right type of file or whatever - and
you'd not need an unlinkat2() with resolve flags).

I think Al needs to ok this kind of change. Maybe you've already
discussed it with him and I just missed it.

            Linus