Restore the ability to mount procfs and fdescfs filesystems via the

mount(2) system call:

  * Add cmount hook to fdescfs and pseudofs (and, by extension, procfs and
    linprocfs).  This (mostly) restores the ability to mount these
    filesystems using the old mount(2) system call (see below for the
    rest of the fix).

  * Remove not-NULL check for the data argument from the mount(2) entry
    point.  Per the mount(2) man page, it is up to the individual
    filesystem being mounted to verify data.  Or, in the case of procfs,
    etc. the filesystem is free to ignore the data parameter if it does
    not use it.  Enforcing data to be not-NULL in the mount(2) system call
    entry point prevented passing NULL to filesystems which ignored the
    data pointer value.  Apparently, passing NULL was common practice
    in such cases, as even our own mount_std(8) used to do it in the
    pre-nmount(2) world.

All userland programs in the tree were converted to nmount(2) long ago,
but I've found at least one external program which broke due to this
(presumably unintentional) mount(2) API change.  One could argue that
external programs should also be converted to nmount(2), but then there
isn't much point in keeping the mount(2) interface for backward
compatibility if it isn't backward compatible.

3 files changed, 24 insertions, 0 deletions

diff --git a/sys/fs/fdescfs/fdesc_vfsops.c b/sys/fs/fdescfs/fdesc_vfsops.c
index c175d0a..6356147 100644
--- a/sys/fs/fdescfs/fdesc_vfsops.c
+++ b/sys/fs/fdescfs/fdesc_vfsops.c
@@ -54,12 +54,22 @@
 
 static MALLOC_DEFINE(M_FDESCMNT, "fdesc_mount", "FDESC mount structure");
 
+static vfs_cmount_t	fdesc_cmount;
 static vfs_mount_t	fdesc_mount;
 static vfs_unmount_t	fdesc_unmount;
 static vfs_statfs_t	fdesc_statfs;
 static vfs_root_t	fdesc_root;
 
 /*
+ * Compatibility shim for old mount(2) system call.
+ */
+int
+fdesc_cmount(struct mntarg *ma, void *data, int flags, struct thread *td)
+{
+	return kernel_mount(ma, flags);
+}
+
+/*
  * Mount the per-process file descriptors (/dev/fd)
  */
 static int
@@ -193,6 +203,7 @@ fdesc_statfs(mp, sbp, td)
 }
 
 static struct vfsops fdesc_vfsops = {
+	.vfs_cmount =		fdesc_cmount,
 	.vfs_init =		fdesc_init,
 	.vfs_mount =		fdesc_mount,
 	.vfs_root =		fdesc_root,
diff --git a/sys/fs/pseudofs/pseudofs.c b/sys/fs/pseudofs/pseudofs.c
index 1fe0371..da1758b 100644
--- a/sys/fs/pseudofs/pseudofs.c
+++ b/sys/fs/pseudofs/pseudofs.c
@@ -285,6 +285,15 @@ pfs_mount(struct pfs_info *pi, struct mount *mp, struct thread *td)
 }
 
 /*
+ * Compatibility shim for old mount(2) system call.
+ */
+int
+pfs_cmount(struct mntarg *ma, void *data, int flags, struct thread *td)
+{
+	return kernel_mount(ma, flags);
+}
+
+/*
  * Unmount a pseudofs instance
  */
 int
diff --git a/sys/fs/pseudofs/pseudofs.h b/sys/fs/pseudofs/pseudofs.h
index 8eaa137..b9701bc 100644
--- a/sys/fs/pseudofs/pseudofs.h
+++ b/sys/fs/pseudofs/pseudofs.h
@@ -34,6 +34,7 @@
 /*
  * Opaque structures
  */
+struct mntarg;
 struct mount;
 struct nameidata;
 struct proc;
@@ -199,6 +200,8 @@ struct pfs_node {
  */
 int		 pfs_mount	(struct pfs_info *pi, struct mount *mp,
 				 struct thread *td);
+int		 pfs_cmount	(struct mntarg *ma, void *data, int flags,
+				 struct thread *td);
 int		 pfs_unmount	(struct mount *mp, int mntflags,
 				 struct thread *td);
 int		 pfs_root	(struct mount *mp, int flags,
@@ -251,6 +254,7 @@ _##name##_uninit(struct vfsconf *vfc) {					\
 }									\
 									\
 static struct vfsops name##_vfsops = {					\
+	.vfs_cmount =		pfs_cmount,				\
 	.vfs_init =		_##name##_init,				\
 	.vfs_mount =		_##name##_mount,			\
 	.vfs_root =		pfs_root,				\