linux-kernel - Re: [PATCH v3 3/4] staging: rtl8723bs: use ether_addr_copy() in rtw_macaddr

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Message-ID: <20180627083306.rk3niwazi332rqs5@mwanda>
Date:   Wed, 27 Jun 2018 11:33:07 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Michael Straube <straube.linux@...il.com>
Cc:     Joe Perches <joe@...ches.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devel@...verdev.osuosl.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/4] staging: rtl8723bs: use ether_addr_copy() in
 rtw_macaddr_cfg()

On Tue, Jun 26, 2018 at 10:32:09PM +0200, Michael Straube wrote:
> 
> On 06/26/18 22:17, Joe Perches wrote:
> > On Tue, 2018-06-26 at 21:44 +0200, Michael Straube wrote:
> > > On 06/26/18 19:32, Andy Shevchenko wrote:
> > > > On Tue, Jun 26, 2018 at 11:14 AM, Michael Straube
> > > > <straube.linux@...il.com> wrote:
> > > > > Use ether_addr_copy() instead of memcpy() to copy the mac address.
> > > > > 
> > > > 
> > > > Suggested-by ?
> > > > 
> > > 
> > > I'll add it. Sorry, I was not aware of the Suggested-by tag.
> > > 
> > > > Btw, ensure that the source and destination buffers are aligned to u16
> > > > as required by API.
> > > 
> > > To be honest I'm not sure how to do that excactly.
> > > 
> > > Use __align(2) in the array declarations? e.g.:
> > > 
> > >           u8 mac[ETH_ALEN] __align(2);
> > 
> > All initial function automatics are naturally aligned.
> > > 
> So there is nothing to change? Now I'm confused.

Do not add the __align(2), as Joe says, it's not required.  You just
need to C alignment rules (it's expected/required for this sort of
patch).

Like if you have a struct:

struct foo {
	char a;
	int b;
};

There is going to be a 3 byte gap between a and b because ints are
normally __align(4).  The exception is when the struct is __packed.  So
sizeof(struct foo) in this case is going to be 8.  kmalloc() returns
pointers which are 8 at least byte aligned normally.  See
ARCH_KMALLOC_MINALIGN.  There is one arch where it's 4 byte aligned?

So when you would get things which aren't __align(2) is when you have:

struct bar {
	char a[3];
	u8 mac[ETH_ALEN];
};

Here the struct member before the mac[] is an odd number of char.  Or
when the struct is packed.

regards,
dan carpenter