linux-kernel - Re: [PATCH 1/3] nfsd: fix refcount leak when failing to hash nfsd

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Message-ID: <0b2fe3a733119b84baf0d65c097f4d716d4c9040.camel@kernel.org>
Date: Thu, 11 Jul 2024 14:20:14 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Chuck Lever III <chuck.lever@...cle.com>, Youzhong Yang
 <youzhong@...il.com>
Cc: Neil Brown <neilb@...e.de>, Olga Kornievskaia <kolga@...app.com>, Dai
 Ngo <dai.ngo@...cle.com>, Tom Talpey <tom@...pey.com>, Linux NFS Mailing
 List <linux-nfs@...r.kernel.org>, Linux Kernel Mailing List
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] nfsd: fix refcount leak when failing to hash
 nfsd_file

On Thu, 2024-07-11 at 18:16 +0000, Chuck Lever III wrote:
> 
> 
> > On Jul 11, 2024, at 1:53 PM, Jeff Layton <jlayton@...nel.org>
> > wrote:
> > 
> > On Thu, 2024-07-11 at 13:05 -0400, Youzhong Yang wrote:
> > > Shouldn't we have fh_put(fhp) before 'retry'?
> > > 
> > 
> > A subtle question, actually...
> > 
> > It probably wouldn't hurt to do that, but I don't think it's
> > required.
> > 
> > The main reason we call fh_put is to force a second call to
> > nfsd_set_fh_dentry. IOW, we want to redo the lookup by filehandle
> > and
> > find the inode.
> > 
> > In the EEXIST case, presumably we have found the inode but we raced
> > with another task in setting an nfsd_file for it in the hash.
> > That's
> > different from the case where the thing was unhashed or we got an
> > EOPENSTALE.  So, I think we probably don't require refinding the
> > inode
> > in that case.
> > 
> > More pointedly, I'm not sure this particular case is actually
> > possible.
> > The entries are hashed on the inode pointer value, and we're
> > searching
> > and inserting into the hash under the i_lock.
> > 
> > Chuck, thoughts?
> 
> Is the question whether we want to dput() the dentry that
> is attached to the fhp ? fh_verify's API contract says:
> 
> 310  * Regardless of success or failure of fh_verify(), fh_put()
> should be
> 311  * called on @fhp when the caller is finished with the
> filehandle. 
> 
> It looks like none of nfsd_file_acquire's callers do an
> fh_put() in their error flows.
> 
> But maybe I've misunderstood the issue.
> 

Note that this API is weird and doesn't conform to typical get/put
semantics.

The fhp is instantiated before nfsd_file_do_acquire is called, and all
of the callers that I can see do eventually call fh_put on it. fh_put
is idempotent, so there should be no harm in calling it multiple times.

My question here though was more about this EEXIST case. Should we even
bother checking for that? I don't see how it's possible.

> 
> > > On Wed, Jul 10, 2024 at 9:06 AM Jeff Layton <jlayton@...nel.org>
> > > wrote:
> > > > 
> > > > At this point, we have a new nf that we couldn't properly
> > > > insert
> > > > into
> > > > the hashtable. Just free it before retrying, since it was never
> > > > hashed.
> > > > 
> > > > Fixes: c6593366c0bf ("nfsd: don't kill nfsd_files because of
> > > > lease
> > > > break error")
> > > > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > > > ---
> > > >  fs/nfsd/filecache.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > > index f84913691b78..4fb5e8546831 100644
> > > > --- a/fs/nfsd/filecache.c
> > > > +++ b/fs/nfsd/filecache.c
> > > > @@ -1038,8 +1038,10 @@ nfsd_file_do_acquire(struct svc_rqst
> > > > *rqstp,
> > > > struct svc_fh *fhp,
> > > >         if (likely(ret == 0))
> > > >                 goto open_file;
> > > > 
> > > > -       if (ret == -EEXIST)
> > > > +       if (ret == -EEXIST) {
> > > > +               nfsd_file_free(nf);
> > > >                 goto retry;
> > > > +       }
> > > >         trace_nfsd_file_insert_err(rqstp, inode, may_flags,
> > > > ret);
> > > >         status = nfserr_jukebox;
> > > >         goto construction_err;
> > > > 
> > > > --
> > > > 2.45.2
> > > > 
> > 
> > -- 
> > Jeff Layton <jlayton@...nel.org>
> 
> 
> --
> Chuck Lever
> 
> 

-- 
Jeff Layton <jlayton@...nel.org>