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Message-ID: <21f3580de20445ddd9bdae6eecc316a58b6df97d.camel@kernel.org>
Subject: Re: [PATCH] NFSD: nfsctl: remove read permission of filehandle
From: Jeff Layton <jlayton@kernel.org>
To: "Hanxiao Chen (Fujitsu)" <chenhx.fnst@fujitsu.com>, Chuck Lever
 <chuck.lever@oracle.com>, Neil Brown <neilb@suse.de>, Olga Kornievskaia
 <kolga@netapp.com>, Dai Ngo <Dai.Ngo@oracle.com>, Tom Talpey
 <tom@talpey.com>
Cc: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Date: Wed, 20 Mar 2024 12:27:36 -0400
In-Reply-To: <TYWPR01MB1208550A963443FA4F3FC5335E6332@TYWPR01MB12085.jpnprd01.prod.outlook.com>
References: <20240319104714.1178-1-chenhx.fnst@fujitsu.com>
	 <3dc4e3e0a730795bb803ce4cde66b45ed428765a.camel@kernel.org>
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On Wed, 2024-03-20 at 15:49 +0000, Hanxiao Chen (Fujitsu) wrote:
>=20
> > -----=E9=82=AE=E4=BB=B6=E5=8E=9F=E4=BB=B6-----
> > =E5=8F=91=E4=BB=B6=E4=BA=BA: Jeff Layton <jlayton@kernel.org>
> > =E5=8F=91=E9=80=81=E6=97=B6=E9=97=B4: 2024=E5=B9=B43=E6=9C=8819=E6=97=
=A5 21:09
> > =E6=94=B6=E4=BB=B6=E4=BA=BA: Chen, Hanxiaochenhx.fnst@fujitsu.com>; Chu=
ck Lever
> > <chuck.lever@oracle.com>; Neil Brown <neilb@suse.de>; Olga Kornievskaia
> > <kolga@netapp.com>; Dai Ngo <Dai.Ngo@oracle.com>; Tom Talpey
> > <tom@talpey.com>
> > =E6=8A=84=E9=80=81: linux-nfs@vger.kernel.org
> > =E4=B8=BB=E9=A2=98: Re: [PATCH] NFSD: nfsctl: remove read permission of=
 filehandle
> >=20
> > On Tue, 2024-03-19 at 18:47 +0800, Chen Hanxiao wrote:
> > > As write_filehandle can't accept zero-bytes writting,
> > > remove read permission of /proc/fs/nfsd/filehandle
> > >=20
> > > Signed-off-by: Chen Hanxiao <chenhx.fnst@fujitsu.com>
> ...
> >=20
> >=20
> > I don't think we can do this. mountd does this to get the rootfh for an
> > exported fs and it'll probably start failing if it can't open that file
> > for read:
> >=20
> >     f =3D open("/proc/fs/nfsd/filehandle", O_RDWR);
> >=20
> > That write_filehandle interface sure is weird though. Could we get rid
> > of it by teaching cache_get_filehandle how to use name_to_handle_at
> > instead? That would be a lot cleaner and would get rid of yet another
> > dependency on /proc/fs/nfsd.
>=20
> name_to_handle_at return struct file_handle
> cache_get_filehandle return hexed struct knfsd_fh
>=20
> Could you please give some hints on how to convert from file_handle. f_ha=
ndle to knfsd_fh.fh_raw?
>=20

I started looking at this yesterday. knfsd_fh is mostly constructed in
fh_compose. The exportfs_encode_fh call is in _fh_update. If we look at
the comments over knfsd_fh, it explains pretty well what we need to fill
out:

fh_version : should always be 1
fh_auth_type: always 0
fh_fsid_type: this is the tricky part (see below)
fh_fileid_type: returned by name_to_handle_at
fh_fsid: fsid concatenated with the fileid for the inode

name_to_handle_at should be able to fill out the fileid portion of the
knfsd_fh. The harder part is: how do we get the fh_fsid_type and its
content?

The types are shown here:

* The fsid_type identifies how the filesystem (or export point) is
 *    encoded.
 *  Current values:
 *     0  - 4 byte device id (ms-2-bytes major, ls-2-bytes minor), 4byte in=
ode number
 *        NOTE: we cannot use the kdev_t device id value, because kdev_t.h
 *              says we mustn't.  We must break it up and reassemble.
 *     1  - 4 byte user specified identifier
 *     2  - 4 byte major, 4 byte minor, 4 byte inode number - DEPRECATED
 *     3  - 4 byte device id, encoded for user-space, 4 byte inode number
 *     4  - 4 byte inode number and 4 byte uuid
 *     5  - 8 byte uuid
 *     6  - 16 byte uuid
 *     7  - 8 byte inode number and 16 byte uuid

The kernel decides this in set_version_and_fsid_type, so I suppose we
could try to replicate that logic in userland.

There is the problem though of what to do about exports with the fsid=3D
option set. That's tougher to determine from userland, but maybe we can
somehow get that from the export record?

That's about as far as I got with it yesterday...
--=20
Jeff Layton <jlayton@kernel.org>