rpc: properly check debugfs dentry before using it

debugfs can now report an error code if something went wrong instead of
just NULL.  So if the return value is to be used as a "real" dentry, it
needs to be checked if it is an error before dereferencing it.

This is now happening because of ff9fb72bc077 ("debugfs: return error
values, not NULL"), but why debugfs files are not being created properly
is an older issue, probably one that has always been there and should
probably be looked at...

Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Cc: netdev@vger.kernel.org
Reported-by: David Howells <dhowells@redhat.com>
Tested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 net/sunrpc/debugfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

I can take this through my tree if people don't object, or it can go
through the NFS tree.  It does need to get merged before 5.0-final
though.

I also have a "larger" debugfs cleanup patch for this file, but that's
not really 5.0-final material and I will send it out later.

thanks,

greg k-h

On Tue, 2019-02-12 at 19:27 +0100, Greg Kroah-Hartman wrote:
> debugfs can now report an error code if something went wrong instead of
> just NULL.  So if the return value is to be used as a "real" dentry, it
> needs to be checked if it is an error before dereferencing it.
> 
> This is now happening because of ff9fb72bc077 ("debugfs: return error
> values, not NULL"), but why debugfs files are not being created properly
> is an older issue, probably one that has always been there and should
> probably be looked at...
> 
> Cc: "J. Bruce Fields" <bfields@fieldses.org>
> Cc: Jeff Layton <jlayton@kernel.org>
> Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
> Cc: Anna Schumaker <anna.schumaker@netapp.com>
> Cc: linux-nfs@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Reported-by: David Howells <dhowells@redhat.com>
> Tested-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> ---
>  net/sunrpc/debugfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> I can take this through my tree if people don't object, or it can go
> through the NFS tree.  It does need to get merged before 5.0-final
> though.

I'm planning another bugfixes pull for 5.0, so I can take this patch and send it
with the others this week.

Thanks!
Anna

> 
> I also have a "larger" debugfs cleanup patch for this file, but that's
> not really 5.0-final material and I will send it out later.
> 
> thanks,
> 
> greg k-h
> 
> diff --git a/net/sunrpc/debugfs.c b/net/sunrpc/debugfs.c
> index 45a033329cd4..19bb356230ed 100644
> --- a/net/sunrpc/debugfs.c
> +++ b/net/sunrpc/debugfs.c
> @@ -146,7 +146,7 @@ rpc_clnt_debugfs_register(struct rpc_clnt *clnt)
>         rcu_read_lock();
>         xprt = rcu_dereference(clnt->cl_xprt);
>         /* no "debugfs" dentry? Don't bother with the symlink. */
> -       if (!xprt->debugfs) {
> +       if (IS_ERR_OR_NULL(xprt->debugfs)) {
>                 rcu_read_unlock();
>                 return;
>         }

On Tue, Feb 12, 2019 at 08:52:53PM +0000, Schumaker, Anna wrote:
> On Tue, 2019-02-12 at 19:27 +0100, Greg Kroah-Hartman wrote:
> > debugfs can now report an error code if something went wrong instead of
> > just NULL.  So if the return value is to be used as a "real" dentry, it
> > needs to be checked if it is an error before dereferencing it.
> > 
> > This is now happening because of ff9fb72bc077 ("debugfs: return error
> > values, not NULL"), but why debugfs files are not being created properly
> > is an older issue, probably one that has always been there and should
> > probably be looked at...
> > 
> > Cc: "J. Bruce Fields" <bfields@fieldses.org>
> > Cc: Jeff Layton <jlayton@kernel.org>
> > Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
> > Cc: Anna Schumaker <anna.schumaker@netapp.com>
> > Cc: linux-nfs@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Reported-by: David Howells <dhowells@redhat.com>
> > Tested-by: David Howells <dhowells@redhat.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > 
> > ---
> >  net/sunrpc/debugfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > I can take this through my tree if people don't object, or it can go
> > through the NFS tree.  It does need to get merged before 5.0-final
> > though.
> 
> I'm planning another bugfixes pull for 5.0, so I can take this patch and send it
> with the others this week.

Wonderful, thanks for doing this, and sorry for the regression.

greg k-h