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Message-ID: <d225561221f558fe917e5554102394ce778a3758.camel@kernel.org>
Subject: Re: [PATCH RFC] fhandle: expose u64 mount id to name_to_handle_at(2)
From: Jeff Layton <jlayton@kernel.org>
To: Christian Brauner <brauner@kernel.org>, Aleksa Sarai <cyphar@cyphar.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>, Chuck
 Lever <chuck.lever@oracle.com>, Amir Goldstein <amir73il@gmail.com>,
 Alexander Aring <alex.aring@gmail.com>, linux-fsdevel@vger.kernel.org, 
 linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org
Date: Tue, 21 May 2024 10:27:23 -0400
In-Reply-To: <20240521-patentfrei-weswegen-0395678c9f9a@brauner>
References: <20240520-exportfs-u64-mount-id-v1-1-f55fd9215b8e@cyphar.com>
	 <20240521-verplanen-fahrschein-392a610d9a0b@brauner>
	 <20240521-patentfrei-weswegen-0395678c9f9a@brauner>
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On Tue, 2024-05-21 at 16:11 +0200, Christian Brauner wrote:
> On Tue, May 21, 2024 at 03:46:06PM +0200, Christian Brauner wrote:
> > On Mon, May 20, 2024 at 05:35:49PM -0400, Aleksa Sarai wrote:
> > > Now that we have stabilised the unique 64-bit mount ID interface in
> > > statx, we can now provide a race-free way for name_to_handle_at(2) to
> > > provide a file handle and corresponding mount without needing to worr=
y
> > > about racing with /proc/mountinfo parsing.
> > >=20
> > > As with AT_HANDLE_FID, AT_HANDLE_UNIQUE_MNT_ID reuses a statx AT_* bi=
t
> > > that doesn't make sense for name_to_handle_at(2).
> > >=20
> > > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > > ---
> >=20
> > So I think overall this is probably fine (famous last words). If it's
> > just about being able to retrieve the new mount id without having to
> > take the hit of another statx system call it's indeed a bit much to
> > add a revised system call for this. Althoug I did say earlier that I
> > wouldn't rule that out.
> >=20
> > But if we'd that then it'll be a long discussion on the form of the new
> > system call and the information it exposes.
> >=20
> > For example, I lack the grey hair needed to understand why
> > name_to_handle_at() returns a mount id at all. The pitch in commit
> > 990d6c2d7aee ("vfs: Add name to file handle conversion support") is tha=
t
> > the (old) mount id can be used to "lookup file system specific
> > information [...] in /proc/<pid>/mountinfo".
> >=20
> > Granted, that's doable but it'll mean a lot of careful checking to avoi=
d
> > races for mount id recycling because they're not even allocated
> > cyclically. With lots of containers it becomes even more of an issue. S=
o
> > it's doubtful whether exposing the mount id through name_to_handle_at()
> > would be something that we'd still do.
> >=20
> > So really, if this is just about a use-case where you want to spare the
> > additional system call for statx() and you need the mnt_id then
> > overloading is probably ok.
> >=20
> > But it remains an unpleasant thing to look at.
>=20
> And I'd like an ok from Jeff and Amir if we're going to try this. :)

I don't have strong feelings about it other than "it looks sort of
ugly", so I'm OK with doing this.

I suspect we will eventually need name_to_handle_at2, or something
similar, as it seems like we're starting to grow some new use-cases for
filehandles, and hitting the limits of the old syscall. I don't have a
good feel for what that should look like though, so I'm happy to put
that off for a while.
--=20
Jeff Layton <jlayton@kernel.org>