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From: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To: Aakash Hemadri <aakashhemadri123@gmail.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Larry Finger <Larry.Finger@lwfinger.net>,
	Phillip Potter <phil@philpotter.co.uk>,
	linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/5] staging: r8188eu: restricted __be16 degrades to int
Date: Fri, 20 Aug 2021 17:10:30 +0200	[thread overview]
Message-ID: <1777630.LqDDHREl4S@localhost.localdomain> (raw)
In-Reply-To: <YR6S7MfxpXpQFl9d@kroah.com>

On Thursday, August 19, 2021 7:20:44 PM CEST Greg Kroah-Hartman wrote:
> On Thu, Aug 19, 2021 at 01:47:56PM +0530, Aakash Hemadri wrote:
> > Fix sparse warning:
> > > rtw_br_ext.c:839:70: warning: restricted __be16 degrades to integer
> > > rtw_br_ext.c:845:70: warning: invalid assignment: |=
> > > rtw_br_ext.c:845:70:    left side has type unsigned short
> > > rtw_br_ext.c:845:70:    right side has type restricted __be16
> > 
> > dhcp->flag is u16, remove htons() as __be16 degrades.
> 
> Um, are you sure?
> 
> > 
> > Signed-off-by: Aakash Hemadri <aakashhemadri123@gmail.com>
> > ---
> >  drivers/staging/r8188eu/core/rtw_br_ext.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/staging/r8188eu/core/rtw_br_ext.c b/drivers/staging/r8188eu/core/rtw_br_ext.c
> > index d4acf02ca64f..14b2935cab98 100644
> > --- a/drivers/staging/r8188eu/core/rtw_br_ext.c
> > +++ b/drivers/staging/r8188eu/core/rtw_br_ext.c
> > @@ -674,13 +674,13 @@ void dhcp_flag_bcast(struct adapter *priv, struct sk_buff *skb)
> >  					u32 cookie = dhcph->cookie;
> >  
> >  					if (cookie == DHCP_MAGIC) { /*  match magic word */
> > -						if (!(dhcph->flags & htons(BROADCAST_FLAG))) {
> > +						if (!(dhcph->flags & BROADCAST_FLAG)) {
> 
> So you now just ignore the fact that the code used to properly check
> BROADCAST_FLAG being in big endian mode, and now you assume it is native
> endian?
> 
> Why is this ok?  Did you test this?
> 
> thanks,
> 
> greg k-h
> 
Aakash,

Building on the objections you had from Greg I suggest that, before attempting 
anew to address problems like these, you get a better understanding of the topics of 
native and network endianness and of the API that (conditionally) swap bytes 
in a variable between little endian and big endian representation.

To start with, please note that the following code leads to tests for "v.vub[0] == 0xDD" 
which is true on little endian architectures while "v.vub[0] == 0xAA" is true on big 
endian ones...

union {
        u32 vud;
        u8 vub[4];
} v;

v.vud = 0xAABBCCDD;

Also note that API like cpu_to_be32(), htonl(), be32_to_cpu(), ntohl, and the likes are 
used to (conditionally) swap bytes (i.e., change the arrangement of the bytes in a 
multi-bytes variable).

Casts have very different purposes and usage patterns and, above all, they cannot 
magically change the endianness of a variable.

Regards,

Fabio



  reply	other threads:[~2021-08-20 15:10 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-19  8:17 [PATCH v2 0/5] staging: r8188eu: fix sparse warnings Aakash Hemadri
2021-08-19  8:17 ` [PATCH v2 1/5] staging: r8188eu: restricted __be16 degrades to int Aakash Hemadri
2021-08-19  8:17 ` [PATCH v2 2/5] staging: r8188eu: cast to restricted __be32 Aakash Hemadri
2021-08-19 12:58   ` Fabio M. De Francesco
2021-08-19 17:19   ` Greg Kroah-Hartman
2021-08-20 11:40     ` Aakash Hemadri
2021-08-20 21:44   ` Larry Finger
2021-08-21 14:21     ` Aakash Hemadri
2021-08-22 21:30     ` David Laight
2021-08-23  8:26       ` Aakash Hemadri
2021-08-23 14:19       ` Larry Finger
2021-08-19  8:17 ` [PATCH v2 3/5] staging: r8188eu: incorrect type in csum_ipv6_magic Aakash Hemadri
2021-08-20 21:38   ` Larry Finger
2021-08-21 14:19     ` Aakash Hemadri
2021-08-20 21:48   ` Larry Finger
2021-08-19  8:17 ` [PATCH v2 4/5] staging: r8188eu: restricted __be16 degrades to int Aakash Hemadri
2021-08-19 17:20   ` Greg Kroah-Hartman
2021-08-20 15:10     ` Fabio M. De Francesco [this message]
2021-08-21 14:18       ` Aakash Hemadri
2021-08-19  8:17 ` [PATCH v2 5/5] staging: r8188eu: incorrect type in assignment Aakash Hemadri

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