Re: [PATCH v3] nfsd: don't hand out write delegations on O_WRONLY opens

[NeilBrown]

On Thu, 03 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 v3:
> - add find_rw_file helper to ensure spinlock is taken appropriately
> - refine comments over conditionals
> - Link to v2: https://lore.kernel.org/r/20230801-wdeleg-v2-1-20c14252bab4@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 | 48 +++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 37 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index ef7118ebee00..c551784d108a 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -649,6 +649,18 @@ find_readable_file(struct nfs4_file *f)
>  	return ret;
>  }
>  
> +static struct nfsd_file *
> +find_rw_file(struct nfs4_file *f)
> +{
> +	struct nfsd_file *ret;
> +
> +	spin_lock(&f->fi_lock);
> +	ret = nfsd_file_get(f->fi_fds[O_RDWR]);
> +	spin_unlock(&f->fi_lock);
> +
> +	return ret;
> +}
> +
>  struct nfsd_file *
>  find_any_file(struct nfs4_file *f)
>  {
> @@ -5449,7 +5461,7 @@ 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 +5473,35 @@ 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. RFC8881 section 10.4 says:
> +	 *
> +	 *  "An OPEN_DELEGATE_WRITE delegation allows the client to handle,
> +	 *   on its own, all opens."
> +	 *
> +	 * Furthermore the client can use a write delegationf or most read
> +	 * operations as well, so we require a O_RDWR file here.
> +	 *
> +	 * Only a write delegation in the case of a BOTH open, and ensure
> +	 * we get the O_RDWR descriptor.
> +	 */

This comment isn't working for me, and it isn't just the need for
    s/f / f/
Neither the "Furthermore" or the "Only a" seem to make sense.

I think the key take away from the RFC quote is "all opens" and that
implies "opens for read".  i.e. all delegations imply read access.
So I would then start the code with

    if (!(open->op_share_access & NFS4_SHARE_ACCESS_READ)) 
         return ERR_PTR(-EACCES);

then choose between readable and rw.
So the comment would say:

	 * RFC8881 section 10.4 says:
	 *
	 *  "An OPEN_DELEGATE_READ delegation allows a client to handle, 
	 *   on its own, requests to open a file for reading ...."
	 * and
	 *  "An OPEN_DELEGATE_WRITE delegation allows the client to handle,
	 *   on its own, all opens."
	 * and as "all" includes "for reading", any delegation must
	 * allow reading.  So if the original access is write-only we
	 * do not return a delegation, otherwise we require at least
	 * "readable", to return a DELGATE_READ and "rw" to return
	 * DELEGATE_WRITE which we only try if the original open
	 * requested write access.

Code looks good, though I find the growth of find_foo_file APIs
aesthetically unpleasant. 
NeilBrown


> +	if ((open->op_share_access & NFS4_SHARE_ACCESS_BOTH) == NFS4_SHARE_ACCESS_BOTH) {
> +		nf = find_rw_file(fp);
>  		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>
> 
>