On 24/04/04 12:31, Arnd Bergmann wrote:
> On Wed, Apr 3, 2024, at 23:00, Daniel Borkmann wrote:
> > On 4/3/24 10:09 PM, Arnd Bergmann wrote:
> >> On Wed, Apr 3, 2024, at 14:33, Anton Protopopov wrote:
> >>>
> >>> Declare this inner union as __attribute__((packed, aligned(2))) such
> >>> that it always is of size 2 and is aligned to 16 bits.
> >>
> >> I think you probably want 32-bit alignment for the structure,
> >> to keep the ABI unchanged on all other architectures.
> >
> > Fwiw, on x86 nothing should change on this regard, see below pahole dump
> > before/after. I think similar might be true for other archs as otherwise
> > we should have seen a kbuild bot complaint on hitting the size assert.
>
> It's not the structure layout that changes, just its alignment.
> Of course this is unlikely to cause actual bugs, but if there there
> is no real need to change it, I would leave the alignment the same
> as before.
I think the struct will now be automatically 4-byte aligned, as it
has the following layout:
struct {
u8 a;
u8 b;
u16 c;
u16 d;
union { u16 e; u16 f; } __aligned__(2);
...
};
So if the union is 2-byte aligned, then the struct is automatically
4-byte aligned, because its address is 6 bytes less than the address
of the union. In fact, as Daniel posted above, pahole shows that the
struct actually has __aligned__(4) attribute in the patched version.
I can add explicit __aligned__(4) to make this clear.
> Arnd
