Re: [PATCH v2] nfsd: don't hand out write delegations on O_WRONLY opens

[Chuck Lever]

On Wed, Aug 02, 2023 at 08:26:15AM +1000, NeilBrown wrote:
> On Tue, 01 Aug 2023, Jeff Layton wrote:
> > I noticed that xfstests generic/001 was failing against linux-next nfsd.
> > 
> > The client would request a OPEN4_SHARE_ACCESS_WRITE open, and the server
> > would hand out a write delegation. The client would then try to use that
> > write delegation as the source stateid in a COPY or CLONE operation, and
> > the server would respond with NFS4ERR_STALE.
> > 
> > The problem is that the struct file associated with the delegation does
> > not necessarily have read permissions. It's handing out a write
> > delegation on what is effectively an O_WRONLY open. RFC 8881 states:
> > 
> >  "An OPEN_DELEGATE_WRITE delegation allows the client to handle, on its
> >   own, all opens."
> > 
> > Given that the client didn't request any read permissions, and that nfsd
> > didn't check for any, it seems wrong to give out a write delegation.
> > 
> > Only hand out a write delegation if we have a O_RDWR descriptor
> > available. If it fails to find an appropriate write descriptor, go
> > ahead and try for a read delegation if NFS4_SHARE_ACCESS_READ was
> > requested.
> > 
> > This fixes xfstest generic/001.
> > 
> > Closes: https://bugzilla.linux-nfs.org/show_bug.cgi?id=412
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> > Changes in v2:
> > - Rework the logic when finding struct file for the delegation. The
> >   earlier patch might still have attached a O_WRONLY file to the deleg
> >   in some cases, and could still have handed out a write delegation on
> >   an O_WRONLY OPEN request in some cases.
> > ---
> >  fs/nfsd/nfs4state.c | 29 ++++++++++++++++++-----------
> >  1 file changed, 18 insertions(+), 11 deletions(-)
> > 
> > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > index ef7118ebee00..e79d82fd05e7 100644
> > --- a/fs/nfsd/nfs4state.c
> > +++ b/fs/nfsd/nfs4state.c
> > @@ -5449,7 +5449,7 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
> >  	struct nfs4_file *fp = stp->st_stid.sc_file;
> >  	struct nfs4_clnt_odstate *odstate = stp->st_clnt_odstate;
> >  	struct nfs4_delegation *dp;
> > -	struct nfsd_file *nf;
> > +	struct nfsd_file *nf = NULL;
> >  	struct file_lock *fl;
> >  	u32 dl_type;
> >  
> > @@ -5461,21 +5461,28 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
> >  	if (fp->fi_had_conflict)
> >  		return ERR_PTR(-EAGAIN);
> >  
> > -	if (open->op_share_access & NFS4_SHARE_ACCESS_WRITE) {
> > -		nf = find_writeable_file(fp);
> > +	/*
> > +	 * Try for a write delegation first. We need an O_RDWR file
> > +	 * since a write delegation allows the client to perform any open
> > +	 * from its cache.
> > +	 */
> > +	if ((open->op_share_access & NFS4_SHARE_ACCESS_BOTH) == NFS4_SHARE_ACCESS_BOTH) {
> > +		nf = nfsd_file_get(fp->fi_fds[O_RDWR]);
> 
> This doesn't take fp->fi_lock before accessing ->fi_fds[], while the
> find_readable_file() call below does.

Note that the code it replaces (find_writeable_file) takes the fi_lock,
so that seems like an important omission.

I noticed this earlier, but I was anxious to test whether this fix is
on the right path. So far, NFSv4.2 behavior seems much improved. And,
I like the new comments.


> This inconsistency suggests a bug?
> 
> Maybe the provided API is awkward.  Should we have 
> find_suitable_file() and find_suitable_file_locked()
> that gets passed an nfs4_file and an O_MODE?
> It tries the given mode, then O_RDWR
> 
> NeilBrown
> 
> 
> >  		dl_type = NFS4_OPEN_DELEGATE_WRITE;
> > -	} else {
> > +	}
> > +
> > +	/*
> > +	 * If the file is being opened O_RDONLY or we couldn't get a O_RDWR
> > +	 * file for some reason, then try for a read deleg instead.
> > +	 */
> > +	if (!nf && (open->op_share_access & NFS4_SHARE_ACCESS_READ)) {
> >  		nf = find_readable_file(fp);
> >  		dl_type = NFS4_OPEN_DELEGATE_READ;
> >  	}
> > -	if (!nf) {
> > -		/*
> > -		 * We probably could attempt another open and get a read
> > -		 * delegation, but for now, don't bother until the
> > -		 * client actually sends us one.
> > -		 */
> > +
> > +	if (!nf)
> >  		return ERR_PTR(-EAGAIN);
> > -	}
> > +
> >  	spin_lock(&state_lock);
> >  	spin_lock(&fp->fi_lock);
> >  	if (nfs4_delegation_exists(clp, fp))
> > 
> > ---
> > base-commit: a734662572708cf062e974f659ae50c24fc1ad17
> > change-id: 20230731-wdeleg-bbdb6b25a3c6
> > 
> > Best regards,
> > -- 
> > Jeff Layton <jlayton@xxxxxxxxxx>
> > 
> > 
> 

-- 
Chuck Lever