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Date:   Tue, 5 Jun 2018 11:53:04 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     Thibaut Robert <thibaut.robert@gmail.com>
Cc:     devel@driverdev.osuosl.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-wireless@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ganesh Krishna <ganesh.krishna@microchip.com>,
        Aditya Shankar <aditya.shankar@microchip.com>
Subject: Re: [PATCH 1/2] staging: wilc1000: fix some endianness sparse
 warnings
Message-ID: <20180605085304.jycb5l3gomwlujq5@mwanda>
References: <20180529191143.13081-1-thibaut.robert@gmail.com>
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 <20180605073631.cvp2iuef4yb636jz@mwanda>
 <20180605083325.GA6965@L80496>
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On Tue, Jun 05, 2018 at 10:33:25AM +0200, Thibaut Robert wrote:
> Le mardi 05 juin 2018 ? 10:36:31 (+0300), Dan Carpenter a ?crit :
> > On Mon, Jun 04, 2018 at 09:32:50PM +0200, Thibaut Robert wrote:
> > > Le mercredi 30 mai 2018 ? 14:17:25 (+0300), Dan Carpenter a ?crit :
> > > > On Tue, May 29, 2018 at 09:11:43PM +0200, Thibaut Robert wrote:
> > > > > diff --git a/drivers/staging/wilc1000/wilc_wfi_cfgoperations.c b/drivers/staging/wilc1000/wilc_wfi_cfgoperations.c
> > > > > index e248702ee519..745bf5ca2622 100644
> > > > > --- a/drivers/staging/wilc1000/wilc_wfi_cfgoperations.c
> > > > > +++ b/drivers/staging/wilc1000/wilc_wfi_cfgoperations.c
> > > > > @@ -1431,7 +1431,7 @@ void wilc_wfi_p2p_rx(struct net_device *dev, u8 *buff, u32 size)
> > > > >  
> > > > >  	freq = ieee80211_channel_to_frequency(curr_channel, NL80211_BAND_2GHZ);
> > > > >  
> > > > > -	if (!ieee80211_is_action(buff[FRAME_TYPE_ID])) {
> > > > > +	if (!ieee80211_is_action(cpu_to_le16(buff[FRAME_TYPE_ID]))) {
> > > > 
> > > > "buff" comes from the network, it's going to be little endian, not cpu
> > > > endian.  The rest of the function treats it as CPU endian but I'm pretty
> > > > sure it's wrong...
> > > buff comes from the network but we are looking at single byte here.
> > > ieee80211_is_action expects an le16, so we I added this to extend an u8
> > > to an le16. Is this incorrect ?
> > > 
> > > Or maybe we the buff has the second byte ? but that I can't tell. 
> > 
> > You raise a good point that I hadn't seen. The original code is clearly
> > buggy.  But your fix isn't correct either...  The other thing to
> > consider is that cpu_to_le16() is basically a cast to u16 on x86 so it's
> > a no-op here.
> The sparse warning is clearly spotting a real issue. I tried to at least
> have big endian handle correctly the 0-255 case. I am willing to drop
> the change (since I agree it's not very satisfying and will mask an issue), 
> but may I ask you to explain how it is wrong ? How would you correctly expand 
> an u8 to __le16 ? I think in big endian we need to swap the bytes.

You're right that on big endian we would need to swap the bytes and
cpu_to_le16() does that.

regards,
dan carpenter