Re: [PATCH] nfsd: special case truncates some more

[bfields@fieldses.org (J. Bruce Fields)]

Thanks!  I split out the cleanup into a separate patch just to make sure
I understood what the important change was....  Looks good to me.

--b.

On Mon, Feb 20, 2017 at 07:21:33AM +0100, Christoph Hellwig wrote:
> Both the NFS protocols and the Linux VFS use a setattr operation with a
> bitmap of attributs to set to set various file attributes including the
> file size and the uid/gid.
> 
> The Linux syscalls never mixes size updates with unrelated updates like
> the uid/gid, and some file systems like XFS and GFS2 rely on the fact
> that truncates might not update random other attributes, and many other
> file systems handle the case but do not update the different attributes
> in the same transaction.  NFSD on the other hand passes the attributes
> it gets on the wire more or less directly through to the VFS, leading to
> updates the file systems don't expect.  XFS at least has an assert on
> the allowed attributes, which caught an unusual NFS client setting the
> size and group at the same time.
> 
> To handle this issue properly this splits the notify_change call in
> nfsd_setattr into two separate ones.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Cc: stable@kernel.org
> ---
>  fs/nfsd/vfs.c | 59 +++++++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 37 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 26c6fdb4bf67..3c36ed5a1f07 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -377,7 +377,7 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp, struct iattr *iap,
>  	__be32		err;
>  	int		host_err;
>  	bool		get_write_count;
> -	int		size_change = 0;
> +	bool		size_change = (iap->ia_valid & ATTR_SIZE);
>  
>  	if (iap->ia_valid & (ATTR_ATIME | ATTR_MTIME | ATTR_SIZE))
>  		accmode |= NFSD_MAY_WRITE|NFSD_MAY_OWNER_OVERRIDE;
> @@ -390,11 +390,11 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp, struct iattr *iap,
>  	/* Get inode */
>  	err = fh_verify(rqstp, fhp, ftype, accmode);
>  	if (err)
> -		goto out;
> +		return err;
>  	if (get_write_count) {
>  		host_err = fh_want_write(fhp);
>  		if (host_err)
> -			return nfserrno(host_err);
> +			goto out;
>  	}
>  
>  	dentry = fhp->fh_dentry;
> @@ -405,20 +405,28 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp, struct iattr *iap,
>  		iap->ia_valid &= ~ATTR_MODE;
>  
>  	if (!iap->ia_valid)
> -		goto out;
> +		return 0;
>  
>  	nfsd_sanitize_attrs(inode, iap);
>  
> +	if (check_guard && guardtime != inode->i_ctime.tv_sec)
> +		return nfserr_notsync;
> +
>  	/*
>  	 * The size case is special, it changes the file in addition to the
> -	 * attributes.
> +	 * attributes, and file systems don't expect it to be mixed with
> +	 * "random" attribute changes.  We thus split out the size change
> +	 * into a separate call to ->setattr, and do the rest as a separate
> +	 * setattr call.
>  	 */
> -	if (iap->ia_valid & ATTR_SIZE) {
> +	if (size_change) {
>  		err = nfsd_get_write_access(rqstp, fhp, iap);
>  		if (err)
> -			goto out;
> -		size_change = 1;
> +			return err;
> +	}
>  
> +	fh_lock(fhp);
> +	if (size_change) {
>  		/*
>  		 * RFC5661, Section 18.30.4:
>  		 *   Changing the size of a file with SETATTR indirectly
> @@ -426,29 +434,36 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp, struct iattr *iap,
>  		 *
>  		 * (and similar for the older RFCs)
>  		 */
> -		if (iap->ia_size != i_size_read(inode))
> -			iap->ia_valid |= ATTR_MTIME;
> -	}
> +		struct iattr size_attr = {
> +			.ia_valid	= ATTR_SIZE | ATTR_CTIME | ATTR_MTIME,
> +			.ia_size	= iap->ia_size,
> +		};
>  
> -	iap->ia_valid |= ATTR_CTIME;
> +		host_err = notify_change(dentry, &size_attr, NULL);
> +		if (host_err)
> +			goto out_unlock;
> +		iap->ia_valid &= ~ATTR_SIZE;
>  
> -	if (check_guard && guardtime != inode->i_ctime.tv_sec) {
> -		err = nfserr_notsync;
> -		goto out_put_write_access;
> +		/*
> +		 * Avoid the additional setattr call below if the only other
> +		 * attribute that the client sends is the mtime, as we update
> +		 * it as part of the size change above.
> +		 */
> +		if ((iap->ia_valid & ~ATTR_MTIME) == 0)
> +			goto out_unlock;
>  	}
>  
> -	fh_lock(fhp);
> +	iap->ia_valid |= ATTR_CTIME;
>  	host_err = notify_change(dentry, iap, NULL);
> -	fh_unlock(fhp);
> -	err = nfserrno(host_err);
>  
> -out_put_write_access:
> +out_unlock:
> +	fh_unlock(fhp);
>  	if (size_change)
>  		put_write_access(inode);
> -	if (!err)
> -		err = nfserrno(commit_metadata(fhp));
>  out:
> -	return err;
> +	if (!host_err)
> +		host_err = commit_metadata(fhp);
> +	return nfserrno(host_err);
>  }
>  
>  #if defined(CONFIG_NFSD_V4)
> -- 
> 2.11.0