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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 12 Mar 2020 09:24:49 -0700
Message-ID: <CAHk-=wgu3Wo_xcjXnwski7JZTwQFaMmKD0hoTZ=hqQv3-YojSg@mail.gmail.com>
Subject: Re: [PATCH 01/14] VFS: Add additional RESOLVE_* flags [ver #18]
To:     Stefan Metzmacher <metze@samba.org>
Cc:     David Howells <dhowells@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Ian Kent <raven@themaw.net>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Karel Zak <kzak@redhat.com>, jlayton@redhat.com,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Mar 12, 2020 at 2:08 AM Stefan Metzmacher <metze@samba.org> 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