[ovs-dev] [PATCH 1/5] odp-execute: Fix unaligned eth_addr pointers.

[Ben Pfaff]

On Thu, May 25, 2017 at 01:05:11PM -0700, Joe Stringer wrote:
> On 24 May 2017 at 20:02, Ben Pfaff <blp at ovn.org> wrote:
> > On Tue, May 23, 2017 at 04:02:12PM -0700, Joe Stringer wrote:
> >> Clang 4.0 complains:
> >>
> >> ../lib/odp-execute.c:61:37: error: taking address of packed member 'eth_dst' of
> >> class or structure 'eth_header' may result in an unaligned pointer value
> >>       [-Werror,-Waddress-of-packed-member]
> >>             ether_addr_copy_masked(&eh->eth_src, key->eth_src, mask->eth_src);
> >>                                     ^~~~~~~~~~~
> >> ../lib/odp-execute.c:62:37: error: taking address of packed member 'eth_dst' of
> >> class or structure 'eth_header' may result in an unaligned pointer value
> >>       [-Werror,-Waddress-of-packed-member]
> >>             ether_addr_copy_masked(&eh->eth_dst, key->eth_dst, mask->eth_dst);
> >>
> >> Ethernet source addresses are 48 bits offset into the Ethernet header,
> >> so taking a pointer for this is not guaranteed to be valid on all
> >> architectures. Fix this by referencing the memory direct from the
> >> Ethernet header pointer.
> >>
> >> Signed-off-by: Joe Stringer <joe at ovn.org>
> >
> > I don't understand--why does Clang think that there's something packed
> > here?  I don't see any packed annotation on struct eth_header (and I
> > don't think it needs one).
> 
> https://github.com/openvswitch/ovs/blob/master/lib/packets.h#L398
> 
> I believe that this is the wire-formatted version that we use for
> assembling PDUs for protocols such as STP, so I think it needs to be
> properly packed.

Oh, oops, somehow I was blind to the difference between eth_header and
eth_addr.

But I don't actually see a need to pack this structure.  There's nothing
in it that would cause a compile to insert padding.  It has all 16-bit
members (including nested members), and you never get padding (on normal
architectures) when all the members of a struct have the same size;
otherwise arrays wouldn't work reasonably.

I'll send some patches.