netdev - Re: [PATCH net-next] SUNRPC: drop pointless static qualifier in xdr_get_next_encode

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Message-ID: <20181108172633.GD4947@fieldses.org>
Date:   Thu, 8 Nov 2018 12:26:33 -0500
From:   "bfields@...ldses.org" <bfields@...ldses.org>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     "yuehaibing@...wei.com" <yuehaibing@...wei.com>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH net-next] SUNRPC: drop pointless static qualifier in
 xdr_get_next_encode_buffer()

On Thu, Nov 08, 2018 at 03:13:25AM +0000, Trond Myklebust wrote:
> On Thu, 2018-11-08 at 02:04 +0000, YueHaibing wrote:
> > There is no need to have the '__be32 *p' variable static since new
> > value
> > always be assigned before use it.

Applying for 4.20 and stable, thanks!

> > 
> > Signed-off-by: YueHaibing <yuehaibing@...wei.com>
> > ---
> >  net/sunrpc/xdr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
> > index 2bbb8d3..d80b156 100644
> > --- a/net/sunrpc/xdr.c
> > +++ b/net/sunrpc/xdr.c
> > @@ -546,7 +546,7 @@ void xdr_commit_encode(struct xdr_stream *xdr)
> >  static __be32 *xdr_get_next_encode_buffer(struct xdr_stream *xdr,
> >  		size_t nbytes)
> >  {
> > -	static __be32 *p;
> > +	__be32 *p;
> >  	int space_left;
> >  	int frag1bytes, frag2bytes;
> > 
> 
> Ouch, that's a really nasty bug that could definitely cause corruption
> if you have 2 threads simultaneously calling this function! This really
> deserves to be a stable patch.

Agreed.  Looks like I introduced that in 3.16, over 5 years ago, so I'm
a little surprised not to have seen a bug report that this would
explain.  Maybe it's just that the critical section is only a few lines
of arithemtic at the end of the function.  Also it only gets called when
an xdr reply other than a read reaches the end of a page.  So you'd need
a lot of concurrent READDIRs of large directories or something.  Still,
I'd think it would be possible.....

> Thank you, YueHaibing!
> 
> Bruce, do you want to shepherd this one in?

Yes, I've got 3 bugfixes queued up now, I should send them along later
today or tomorrow.

--b.