struct sockaddr * and alignment

[M. Warner Losh]

In message: <b649e5e1002100148r759f3aacr3d5fcdfb5efd9001 at mail.gmail.com>
            Marius Nünnerich <marius at nuenneri.ch> writes:
: On Tue, Feb 9, 2010 at 18:34, M. Warner Losh <imp at bsdimp.com> wrote:
: > Greetings,
: >
: > I've found a few inconsistencies in the sockaddr stuff in the tree.
: > I'm not sure where to go to get a definitive answer, so I thought I'd
: > start here.
: >
: > I got here looking at the recent wake breakage on mips.  It turns out
: > that the warning was:
: >
: > src/usr.sbin/wake/wake.c: In function 'find_ether':
: > src/usr.sbin/wake/wake.c:123: warning: cast increases required alignment of target type
: >
: > which comes from
: >        sdl = (struct sockaddr_dl *)ifa->ifa_addr;
: >
: > The problem is that on MIPS struct sockaddr * is byte aligned and
: > sockaddr_dl * is word aligned, so the compiler is rightly telling us
: > that there might be a problem here.
: >
: > However, further digging shows that there will never be a problem
: > here with alignment.  struct sockaddr_storage has a int64 in it to
: > force it to be __aligned(8).  So I thought to myself "why don't I just
: > add __aligned(8) to the struct sockaddr definition?"  After all, the
: > kernel goes to great lengths to return data so aligned, and user code
: > also keeps things aligned.
: >
: > Sure enough, that fixes this warning.  Yea.  But, sadly, it causes
: > other problems.  If you look at sbin/atm/atmconfig/natm.c you'll see
: > code like:
: >
: > static void
: > store_route(struct rt_msghdr *rtm)
: > {
: > ...
: >        char *cp
: >        struct sockaddr *sa;
: >        ...
: >
: >        cp = (char *)(rtm + 1);
: > ...
: >                        sa = (struct sockaddr *)cp;
: >                        cp += roundup(sa->sa_len, sizeof(long));
: > ...
: >
: > which breaks because we're now casting from an __aligned(1) char * to
: > an __aligned(8) sockaddr *.
: >
: > And it is only rounding the size of the structure to long, rather than
: > int64 like sockaddr_storage suggests is the proper alignment.  But I
: > haven't looked in the kernel to see if there's an issue there with
: > routing sockets or not.
: >
: > The other extreme is to put __aligned(1) on all the sockaddr_foo
: > structures.  This would solve the compiler warning, but would have a
: > negative effect on performance in accessing these elements (because
: > the compiler would have to generate calls to bcopy or equivalent to
: > access the scalar members that are larger than a byte).   This cure
: > would be worse than the disease.
: >
: > So the question here is "What is the right solution here?"  It has me
: > stumped.  So I dropped WARNS level down from 6 to 3 for wake.c.
: 
: Hi Warner,
: 
: I got into the same kind of trouble when I tried to raise the WARNS
: level above 3 for inetd and others. I guess everything which uses some
: sockaddr casting or (in the case of inetd) some of these macros:
: http://fxr.googlebit.com/source/sys/netinet6/in6.h?v=8-CURRENT#L233
: 
: It's a pity that only this keeps some programs from going to a WARNS level of 6.

Well, if there were some way to tell the compiler "Yes, I know this is
right" then we'd be set.  But I've not found what that way might be.

Warner