Re: [PATCH 8/8] staging: rtl8188eu: use is_broadcast_ether_addr

[Joe Perches]

On Sun, 2018-07-29 at 20:21 +0200, Michael Straube wrote:
> On 07/29/18 19:59, Joe Perches wrote:
> > On Sun, 2018-07-29 at 19:42 +0200, Michael Straube wrote:
> > > On 07/29/18 19:21, Joe Perches wrote:
> > > > On Sun, 2018-07-29 at 19:08 +0200, Michael Straube wrote:
> > > > > Use is_broadcast_ether_addr instead of checking each byte of the
> > > > > address array for 0xff. Shortens the code and improves readability.
> > > > 
> > > > You should show in the commit log that sta_addr is __aligned(2)
> > > > as required by is_broadcast_ether_addr, otherwise you could be
> > > > introducing runtime alignment defects.
> > > > 
> > > 
> > > Ok, sta_addr is used from following structs.
> > > 
> > > struct ieee_param {
> > >           u32 cmd;
> > >           u8 sta_addr[ETH_ALEN];
> > >           union {
> > >           ...
> > >           ...
> > >           }; u
> > > };
> > > 
> > > struct ieee_param_ex {
> > > 	u32 cmd;
> > > 	u8 sta_addr[ETH_ALEN];
> > > 	u8 data[0];
> > > };
> > > 
> > > Well, looking at it now, I'm not sure about the alignment anymore
> > > in the struct that contains the union. Is sta_addr in the first
> > > struct __aligned(2)?
> > > 
> > > Should I include the snippets in the commit message, or is just
> > > writing that sta_addr is __aligned(2) enough? (if it is in the
> > > first case...)
> > 
> > It's enough to just state that the uses are properly aligned
> > as long as you looked and understand that it's required.
> > 
> 
> Ok, thank you.
> 
> I looked at it and understand that it's required.
> But, as mentioned, at second look I'm not sure about the union.
> 
> I guess I need to read a little more about the alignment of unions.
> Any hints are welcomed. :)

The union doesn't matter here.
Only the locations of sta_addr matter.
Both follow a u32, so they are also aligned to a u32.

That is actually not guaranteed by the c standard
as the u8 array could have arbitrary padding bytes
inserted between the u32, but no compiler used by
the kernel does that.