nfsd: Fix NFS access to .zfs/snapshot snapshots

When a process attempts to access a snapshot under
/<dataset>/.zfs/snapshot, the snapshot is automounted.
However, without this patch, the automount does not
set mnt_exjail, which results in the snapshot not being
accessible over NFS.

This patch defines a new function called vfs_exjail_clone()
which sets mnt_exjail from another mount point and
then uses that function to set mnt_exjail in the snapshot
automount.  A separate patch that is currently a pull request
for OpenZFS, calls this function to fix the problem.

PR:	275200
Reviewed by:	markj
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D42672

sys/kern/vfs_mount.c

@@ -3136,6 +3136,41 @@ suspend_all_fs(void)
 	mtx_unlock(&mountlist_mtx);
 }
 
+/*
+ * Clone the mnt_exjail field to a new mount point.
+ */
+void
+vfs_exjail_clone(struct mount *inmp, struct mount *outmp)
+{
+	struct ucred *cr;
+	struct prison *pr;
+
+	MNT_ILOCK(inmp);
+	cr = inmp->mnt_exjail;
+	if (cr != NULL) {
+		crhold(cr);
+		MNT_IUNLOCK(inmp);
+		pr = cr->cr_prison;
+		sx_slock(&allprison_lock);
+		if (!prison_isalive(pr)) {
+			sx_sunlock(&allprison_lock);
+			crfree(cr);
+			return;
+		}
+		MNT_ILOCK(outmp);
+		if (outmp->mnt_exjail == NULL) {
+			outmp->mnt_exjail = cr;
+			atomic_add_int(&pr->pr_exportcnt, 1);
+			cr = NULL;
+		}
+		MNT_IUNLOCK(outmp);
+		sx_sunlock(&allprison_lock);
+		if (cr != NULL)
+			crfree(cr);
+	} else
+		MNT_IUNLOCK(inmp);
+}
+
 void
 resume_all_fs(void)
 {

sys/sys/mount.h

@@ -980,6 +980,9 @@ enum vfs_notify_upper_type {
  * exported vnode operations
  */
 
+/* Define this to indicate that vfs_exjail_clone() exists for ZFS to use. */
+#define	VFS_SUPPORTS_EXJAIL_CLONE	1
+
 int	dounmount(struct mount *, uint64_t, struct thread *);
 
 int	kernel_mount(struct mntarg *ma, uint64_t flags);
@@ -1016,6 +1019,7 @@ int	vfs_setpublicfs			    /* set publicly exported fs */
 	    (struct mount *, struct netexport *, struct export_args *);
 void	vfs_periodic(struct mount *, int);
 int	vfs_busy(struct mount *, int);
+void	vfs_exjail_clone(struct mount *, struct mount *);
 void	vfs_exjail_delete(struct prison *);
 int	vfs_export			 /* process mount export info */
 	    (struct mount *, struct export_args *, bool);