svn commit: r341689 - in head: lib/libc/sys sys/compat/freebsd32 sys/kern sys/sys

[Konstantin Belousov]

On Fri, Dec 07, 2018 at 09:21:34AM -0800, John Baldwin wrote:
> On 12/7/18 7:17 AM, Konstantin Belousov wrote:
> > Author: kib
> > Date: Fri Dec  7 15:17:29 2018
> > New Revision: 341689
> > URL: https://svnweb.freebsd.org/changeset/base/341689
> > 
> > Log:
> >   Add new file handle system calls.
> >   
> >   Namely, getfhat(2), fhlink(2), fhlinkat(2), fhreadlink(2).  The
> >   syscalls are provided for a NFS userspace server (nfs-ganesha).
> >   
> >   Submitted by:	Jack Halford <jack at gandi.net>
> >   Sponsored by:	Gandi.net
> >   Tested by:	pho
> >   Feedback from:	brooks, markj
> >   MFC after:	1 week
> >   Differential revision:	https://reviews.freebsd.org/D18359
> 
> Can this be used to implement 'flink' (create a link to an open file
> descriptor)?  Hmm, it appears so.  It is limited to PRIV_VFS_GETFH at least.
> The getfh(2) manpage notes this explicitly, but the new manpages don't
> appear to.  Even with the PRIV check, I'm still somewhat nervous about what
> flink means for processes running as root that are using Capsicum.  Maybe
> it's ok, but I didn't see any discussion of this in the review.

If the process can execute getfh(2) and fhlink(2), then its privileges
are not much different from the privileges of the in-kernel NFS server.
During the review, I verified that PRIV_VFS_GETFH is checked, and considered
suggesting fine-grainer individual privs for other operations, but decided
that this is not too useful.

That said, how can you translate from file descriptor to file
handle ? E.g. to know (and not guess) the file handle on UFS,
the process must posses PRIV_VFS_GENERATION. If you have
PRIV_VFS_GETFH/PRIV_VFS_GENERATION privs, then you can implement
flink(2) for UFS, but might be that it is even not undesirable.