linux-kernel - [PATCH v2 30/31] nfsd: only allow filesystems that set EXPORT_OP_STABLE

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Message-Id: <20260119-exportfs-nfsd-v2-30-d93368f903bd@kernel.org>
Date: Mon, 19 Jan 2026 11:26:47 -0500
From: Jeff Layton <jlayton@...nel.org>
To: Christian Brauner <brauner@...nel.org>, 
 Alexander Viro <viro@...iv.linux.org.uk>, 
 Chuck Lever <chuck.lever@...cle.com>, NeilBrown <neil@...wn.name>, 
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Cc: David Laight <david.laight.linux@...il.com>, 
 Dave Chinner <david@...morbit.com>, Christoph Hellwig <hch@...radead.org>, 
 linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
 linux-ext4@...r.kernel.org, linux-erofs@...ts.ozlabs.org, 
 linux-xfs@...r.kernel.org, ceph-devel@...r.kernel.org, 
 linux-btrfs@...r.kernel.org, linux-cifs@...r.kernel.org, 
 samba-technical@...ts.samba.org, linux-unionfs@...r.kernel.org, 
 devel@...ts.orangefs.org, ocfs2-devel@...ts.linux.dev, 
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 jfs-discussion@...ts.sourceforge.net, linux-mtd@...ts.infradead.org, 
 gfs2@...ts.linux.dev, linux-f2fs-devel@...ts.sourceforge.net, 
 linux-doc@...r.kernel.org, Jeff Layton <jlayton@...nel.org>
Subject: [PATCH v2 30/31] nfsd: only allow filesystems that set
 EXPORT_OP_STABLE_HANDLES

Some filesystems have grown export operations in order to provide
filehandles for local usage. Some of these filesystems are unsuitable
for use with nfsd, since their filehandles are not stable across
reboots.

In check_export(), check whether EXPORT_OP_STABLE_HANDLES is set
and return -EINVAL if it isn't.

Signed-off-by: Jeff Layton <jlayton@...nel.org>
---
 fs/nfsd/export.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index 2a1499f2ad196a6033787260881e451146283bdc..bc703cf58bfa210c7c57d49f22f15bc10d7cfc91 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -422,13 +422,15 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
 	if (*flags & NFSEXP_V4ROOT)
 		*flags |= NFSEXP_READONLY;
 
-	/* There are two requirements on a filesystem to be exportable.
-	 * 1:  We must be able to identify the filesystem from a number.
+	/* There are four requirements on a filesystem to be exportable:
+	 * 1: We must be able to identify the filesystem from a number.
 	 *       either a device number (so FS_REQUIRES_DEV needed)
 	 *       or an FSID number (so NFSEXP_FSID or ->uuid is needed).
-	 * 2:  We must be able to find an inode from a filehandle.
+	 * 2: We must be able to find an inode from a filehandle.
 	 *       This means that s_export_op must be set.
-	 * 3: We must not currently be on an idmapped mount.
+	 * 3: It must provide stable filehandles.
+	 *       This means that EXPORT_OP_STABLE_HANDLES is set
+	 * 4: We must not currently be on an idmapped mount.
 	 */
 	if (!(inode->i_sb->s_type->fs_flags & FS_REQUIRES_DEV) &&
 	    !(*flags & NFSEXP_FSID) &&
@@ -442,6 +444,11 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
 		return -EINVAL;
 	}
 
+	if (!(inode->i_sb->s_export_op->flags & EXPORT_OP_STABLE_HANDLES)) {
+		dprintk("%s: fs does not provide stable filehandles!\n", __func__);
+		return -EINVAL;
+	}
+
 	if (is_idmapped_mnt(path->mnt)) {
 		dprintk("exp_export: export of idmapped mounts not yet supported.\n");
 		return -EINVAL;

-- 
2.52.0