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

[Omar Sandoval <osandov@osandov.com>]

On Fri, Jan 23, 2015 at 08:07:41AM -0500, Trond Myklebust wrote:
> On Thu, Jan 22, 2015 at 11:07 PM, Omar Sandoval <osandov@osandov.com> wrote:
> > On Mon, Jan 12, 2015 at 11:43:02AM -0800, Omar Sandoval wrote:
> >> 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
> >
> > Hi, Trond,
> >
> > Are you still planning on taking this patch? I didn't see it in your
> > last pull request to Linus.
> 
> I was planning on pushing it in the 3.20 merge window.
> 
> Is there any reason to fasttrack it earlier as an important bugfix? To
> me it seems more like an "assist user to not shoot self in foot" type
> of thing.
> 
> Cheers
>   Trond
> 
> -- 
> Trond Myklebust
> Linux NFS client maintainer, PrimaryData
> trond.myklebust@primarydata.com

Ah, okay, 3.20 should be fine.

Thanks,
-- 
Omar