From: "J. Bruce Fields" Subject: Re: [PATCH 1/4] knfsd: Replace lock_kernel with a mutex for nfsd thread startup/shutdown locking. Date: Wed, 4 Jun 2008 18:41:20 -0400 Message-ID: <20080604224120.GI10362@fieldses.org> References: <1212591796-22144-1-git-send-email-jlayton@redhat.com> <1212591796-22144-2-git-send-email-jlayton@redhat.com> <20080604210235.GC10362@fieldses.org> <20080604172752.31686797@tleilax.poochiereds.net> <20080604215815.GF10362@fieldses.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-nfs@vger.kernel.org, nfsv4@linux-nfs.org, gnb-cP1dWloDopni96+mSzHFpQC/G2K4zDHf@public.gmane.org To: Jeff Layton Return-path: Received: from mail.fieldses.org ([66.93.2.214]:39946 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751945AbYFDWlW (ORCPT ); Wed, 4 Jun 2008 18:41:22 -0400 In-Reply-To: <20080604215815.GF10362@fieldses.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Wed, Jun 04, 2008 at 05:58:15PM -0400, bfields wrote: > On Wed, Jun 04, 2008 at 05:27:52PM -0400, Jeff Layton wrote: > > On Wed, 4 Jun 2008 17:02:35 -0400 > > "J. Bruce Fields" wrote: > > > > > On Wed, Jun 04, 2008 at 11:03:13AM -0400, Jeff Layton wrote: > > > > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c > > > > index 5ac00c4..d601a77 100644 > > > > --- a/fs/nfsd/nfsctl.c > > > > +++ b/fs/nfsd/nfsctl.c > > > ... > > > > @@ -566,14 +574,13 @@ static ssize_t write_versions(struct file *file, char *buf, size_t size) > > > > return len; > > > > } > > > > > > > > -static ssize_t write_ports(struct file *file, char *buf, size_t size) > > > > +static ssize_t __write_ports(struct file *file, char *buf, size_t size) > > > > { > > > > if (size == 0) { > > > > int len = 0; > > > > - lock_kernel(); > > > > + > > > > if (nfsd_serv) > > > > len = svc_xprt_names(nfsd_serv, buf, 0); > > > > - unlock_kernel(); > > > > > > svc_xprt_names() has to be prepared to accept NULL as a first parameter > > > (since we've got nothing here any longer to guarantee that nfsd_serv > > > won't change after we've checked it). And, indeed, it does check for > > > that (with its local copy, which won't change. So that's OK. But then > > > could we just ditch this redundant check here? It's confusing. > > > > > > Oops, but: what happens if something like this races with svc_destroy, > > > so svc_xprt_names() is passed a pointer to freed memory? > > > > > > > We do have a guarantee that nfsd_serv won't change after it's checked > > here. The new nfsd_mutex protects it. write_ports has been renamed to > > __write_ports, and write_ports has been turned into a wrapper that runs > > the entire original function under the nfsd_mutex. We also have nfsd > > hold the nfsd_mutex when svc_exit_thread is called, so svc_destroy > > should also be called while holding it. That should serialize access > > to the nfsd_serv. > > Of course, you're right; thanks for setting me straight! One more random point of confusion: is write_versions racy? It assigns to nfsd_versions, which is used in svc_process() to decide whether a version is supported or not, without doing the adjustment of rq_argp and rq_resp which a comment in write_versions() says is necessary. And there's no locking around the nfsd_serv check there. So in theory could a write_versions() at the wrong time result in an nfsd that accepted nfs versions that it shouldn't (and hence could overflow some buffer)? That'd be a preexisting problem, nothing to do with your work--I was just grepping for uses of nfsd_serv.... --b.