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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Thu, 4 May 2023 14:37:38 +0300
Message-ID: <CAOQ4uxh4Kij7fMyFOMgdnZee5_HcHc9RYTNxvLTBtD-JxpCZwQ@mail.gmail.com>
Subject: Re: [PATCH v2 3/4] exportfs: allow exporting non-decodeable file
 handles to userspace
To:     Christian Brauner <brauner@kernel.org>
Cc:     Jan Kara <jack@suse.cz>, Miklos Szeredi <miklos@szeredi.hu>,
        Jeff Layton <jlayton@kernel.org>,
        Chuck Lever <chuck.lever@oracle.com>,
        linux-fsdevel@vger.kernel.org, linux-unionfs@vger.kernel.org,
        linux-nfs@vger.kernel.org, linux-api@vger.kernel.org,
        Al Viro <viro@zeniv.linux.org.uk>
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On Thu, May 4, 2023 at 2:19=E2=80=AFPM Christian Brauner <brauner@kernel.or=
g> wrote:
>
> On Wed, May 03, 2023 at 07:23:14PM +0200, Jan Kara wrote:
> > On Tue 02-05-23 15:48:16, Amir Goldstein wrote:
> > > Some userspace programs use st_ino as a unique object identifier, eve=
n
> > > though inode numbers may be recycable.
> > >
> > > This issue has been addressed for NFS export long ago using the expor=
tfs
> > > file handle API and the unique file handle identifiers are also expor=
ted
> > > to userspace via name_to_handle_at(2).
> > >
> > > fanotify also uses file handles to identify objects in events, but on=
ly
> > > for filesystems that support NFS export.
> > >
> > > Relax the requirement for NFS export support and allow more filesyste=
ms
> > > to export a unique object identifier via name_to_handle_at(2) with th=
e
> > > flag AT_HANDLE_FID.
> > >
> > > A file handle requested with the AT_HANDLE_FID flag, may or may not b=
e
> > > usable as an argument to open_by_handle_at(2).
> > >
> > > To allow filesystems to opt-in to supporting AT_HANDLE_FID, a struct
> > > export_operations is required, but even an empty struct is sufficient
> > > for encoding FIDs.
> >
> > Christian (or Al), are you OK with sparing one AT_ flag for this
> > functionality? Otherwise the patch series looks fine to me so I'd like =
to
> > queue it into my tree. Thanks!
>
> At first it looked like there are reasons to complain about this on the
> grounds that this seems like a flag only for a single system call. But
> another look at include/uapi/linux/fcntl.h reveals that oh well, the
> AT_* flag namespace already contains system call specific flags.
>
> The overloading of AT_EACCESS and AT_REMOVEDIR as 0x200 is especially
> creative since it doesn't even use an infix like the statx specific
> flags.
>
> Long story short, since there's already overloading of the flag
> namespace happening it wouldn't be unprecedent or in any way wrong if
> this patch just reused the 0x200 value as well.
>

I had considered this myself as well...
Couldn't decide if this was ugly or not.
Obviously, I do not mind which value the flag gets.

> In fact, it might come in handy since it would mean that we have the bit
> you're using right now free for a flag that is meaningful for multiple
> system calls. So something to consider but you can just change that
> in-tree as far as I'm concerned.
>
> All this amounts to a long-winded,
>
> Acked-by: Christian Brauner <brauner@kernel.org>

Thanks,
Amir.