Re: What to do about a few lines in vfs_domount() never executed?

Tracking the commits, it was originally introduced to
sys/kern/vfs_syscalls.c at r22521 [1][2] (Mon Feb 10 02:22:35 1997 by
dyson, submitted by hsu@freebsd.org) and later centralized into
sys/kern/vfs_mount.c at r99264 [2].

But according to the comment above the codes, maybe it would be
intended to block userland programs or ports FS modules setting 
MNT_EXPORTED.

If I'm not mis-understanding, it can be the case when
 *vfs.usermount sysctl is non-zero,
 *underlying FS (to be exported) allows it, and
 *non-root user tries to mount the FS via NFS.


[1]
https://svnweb.freebsd.org/base/head/sys/kern/vfs_syscalls.c?revision=22521&view=markup&pathrev=99264

[2]
https://svnweb.freebsd.org/base/head/sys/kern/vfs_syscalls.c?r1=22520&r2=22521&pathrev=99264&

[3]
https://cgit.freebsd.org/src/commit/sys/kern/vfs_mount.c?id=2b4edb69f1ef62fc38b02ac22b0a3ac09e43fa77


On Tue, 13 Dec 2022 14:19:39 -0800
Rick Macklem <rick.macklem@gmail.com> wrote:

> Hi,
> 
> While working on getting mountd/nfsd to run in a vnet
> prison, I came across the following lines near the
> beginning of vfs_domount() in sys/kern/vfs_mount.c:
> 
> if (fsflags & MNT_EXPORTED) {
>      error = priv_check(td, PRIV_VFS_MOUNT_EXPORTED);
>      if (error)
>            return (error);
> }
> 
> #1 - Since MNT_EXPORTED is never set in fsflags, this code never
>      gets executed.
>      --> I am asking what to do with the above code, since that
>          changes for the patch that allows mountd to run in a vnet
>          prison.
> #2 - priv_check(td, PRIV_VFS_MOUNT_EXPORTED) always returns 0
>      because nothing in sys/kern/kern_priv.c checks
>      PRIV_VFS_MOUNT_EXPORTED.
> 
> I don't know what the original author's thinking was w.r.t. this.
> Setting exports already checks that the mount operation can be
> done by the requestor.
> 
> So, what do you think should be done with the above code snippet?
> - Consider it cruft and delete it.
> - Try and figure out what PRIV_VFS_MOUNT_EXPORTED should check?
> - Leave it as is. After the patch that allows mountd to run in
>   a vnet prison, MNT_EXPORTED will be set in fsflags, but the
>   priv_check() call will just return 0. (A little overhead,
>   but otherwise no semantics change.)
> 
> rick


-- 
Tomoaki AOKI    <junchoon@dec.sakura.ne.jp>