Re: [PATCH RESEND] nfs: prevent truncate on active swapfile

[Omar Sandoval <osandov@osandov.com>]

On Sat, Jan 10, 2015 at 05:08:03PM -0500, Trond Myklebust wrote:
> Hi Omar,
> 
> On Thu, Jan 8, 2015 at 4:18 AM, Omar Sandoval <osandov@osandov.com> wrote:
> > Most filesystems prevent truncation of an active swapfile by way of
> > inode_newsize_ok, called from inode_change_ok. NFS doesn't call either
> > from nfs_setattr, presumably because most of these checks are expected
> > to be done server-side. However, the IS_SWAPFILE check can only be done
> > client-side, and truncating a swapfile can't possibly be good.
> >
> > Signed-off-by: Omar Sandoval <osandov@osandov.com>
> > ---
> > Hi, Trond,
> >
> > Now that the holidays are over, could you take a look at this? It was
> > generated against v3.19-rc3.
> >
> > Thanks!
> >
> >  fs/nfs/inode.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
> > index 4bffe63..9205513 100644
> > --- a/fs/nfs/inode.c
> > +++ b/fs/nfs/inode.c
> > @@ -506,10 +506,15 @@ nfs_setattr(struct dentry *dentry, struct iattr *attr)
> >                 attr->ia_valid &= ~ATTR_MODE;
> >
> >         if (attr->ia_valid & ATTR_SIZE) {
> > +               loff_t i_size;
> > +
> >                 BUG_ON(!S_ISREG(inode->i_mode));
> >
> > -               if (attr->ia_size == i_size_read(inode))
> > +               i_size = i_size_read(inode);
> > +               if (attr->ia_size == i_size)
> >                         attr->ia_valid &= ~ATTR_SIZE;
> > +               else if (attr->ia_size < i_size && IS_SWAPFILE(inode))
> > +                       return -ETXTBSY;
> >         }
> >
> >         /* Optimization: if the end result is no change, don't RPC */
> > --
> > 2.2.1
> >
> 
> I agree that truncating a swap file is bad, however as you point out,
> this really only addresses the case on the client that knows about
> this being a swap file.
> I'll take the patch,

Thanks, I appreciate it.

> but I'm wondering if we couldn't do better in the
> case where we're using NFSv4 by using share deny modes (which are
> enforced by the server). The problem is that there appears to be
> nothing in swapon() that tells the filesystem this is an open of a
> swap file...

Yeah, it would be nice for completeness to prevent one client from
truncating another client's swapfile. However, I'd hope that anyone
using swap-over-NFS on a shared NFS mount would take the necessary
precautions in terms of permissions, etc. to prevent someone from doing
that. Also, since the failure mode of truncating an NFS swapfile is a
corrupt swapfile rather than a corrupt filesystem (like on a local
filesystem), it's probably okay to just deal with the low-hanging fruit
for now.

Thanks!

> 
> Cheers
>   Trond
> -- 
> Trond Myklebust
> Linux NFS client maintainer, PrimaryData
> trond.myklebust@primarydata.com

-- 
Omar