I made a quick and dirty sparse patch to check for this.  I don't think
I will bother trying to send it to sparse upstream, but you can if you
want to.

It found 289 unions which might need a __packed added.  The lustre
unions were not in my allmodconfig so they're not listed.

Perhaps there could be a command line option or a pragma so that unions
will work in the kernel.  We don't care about linking to outside
libraries.

regards,
dan carpenter

diff --git a/symbol.c b/symbol.c
index ebba56deaf94..596e47883aad 100644
--- a/symbol.c
+++ b/symbol.c
@@ -187,6 +187,12 @@ static struct symbol * examine_struct_union_type(struct symbol *sym, int advance
 		bit_size = (bit_size + bit_align) & ~bit_align;
 	}
 	sym->bit_size = bit_size;
+
+	if (!advance && (info.bit_size / 8) % 4) {
+		int pad = 4 - (info.bit_size / 8) % 4;
+		warning(sym->pos, "'%s' union will be padded with %d bytes unless __attribute__((packed)).",  sym->ident ? sym->ident->name: "<null>", pad);
+	}
+
 	return sym;
 }