linux-kernel - Re: [PATCH 2/2] staging: emxx

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Date:	Sat, 4 Apr 2015 18:07:40 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Julia Lawall <Julia.Lawall@...6.fr>, devel@...verdev.osuosl.org,
	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] staging: emxx_udc: test returned value

On Sat, Apr 04, 2015 at 06:59:58PM +0300, Dan Carpenter wrote:
> This is a clever Coccinelle check.  :)
> 
> On Sat, Apr 04, 2015 at 04:59:30PM +0200, Julia Lawall wrote:
> > Put NULL test on the result of the previous call instead on one of its
> > arguments.  A simplified version of the semantic match that finds this
> > problem is as follows (http://coccinelle.lip6.fr/):
> > 
> > // <smpl>
> > r@
> > expression *e1;
> > expression *e2;
> > identifier f;
> > statement S1,S2;
> > @@
> > 
> > e1 = f(...,e2,...);
> > (
> > if (e1 == NULL || ...) S1 else S2
> > |
> > *if (e2 == NULL || ...) S1 else S2
> > )
> > // </smpl>
> > 
> > Signed-off-by: Julia Lawall <Julia.Lawall@...6.fr>
> > 
> > ---
> >  drivers/staging/emxx_udc/emxx_udc.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/staging/emxx_udc/emxx_udc.c b/drivers/staging/emxx_udc/emxx_udc.c
> > index fbf82bc..7de1e9e 100644
> > --- a/drivers/staging/emxx_udc/emxx_udc.c
> > +++ b/drivers/staging/emxx_udc/emxx_udc.c
> > @@ -2998,7 +2998,7 @@ static void  nbu2ss_ep_fifo_flush(struct usb_ep *_ep)
> >  	}
> >  
> >  	ep = container_of(_ep, struct nbu2ss_ep, ep);
> > -	if (!_ep) {
> > +	if (!ep) {
> >  		pr_err("udc: %s, bad ep\n", __func__);
> >  		return;
> 
> The better thing to do here is to remove the condition.  container_of()
> takes an pointer and subtracts an offset so testing the return value
> is normally meaningless.
> 
> Sometimes (and in this case) the offset is zero.  In that situation it
> might make sense to check the return value but it's an ugly thing to
> do because if we re-order the nbu2ss_ep struct layout so "ep" isn't the
> first member then it breaks.
> 
> In this case we already verified that "_ep" is non-NULL so the check
> isn't needed.

This will teach me to read ahead in an email thread... :)
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