On Mon, Apr 12, 2021 at 12:54 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: David Laight > Sent: 12 April 2021 10:37
> ...
> > I'm guessing that compat_pid_t is 16 bits?
> > So the native 32bit version has an unnamed 2 byte structure pad.
> > The 'packed' removes this pad from the compat structure.
> >
> > AFAICT (apart from mips) the __ARCH_COMPAT_FLOCK_PAD is just
> > adding an explicit pad for the implicit pad the compiler
> > would generate because compat_pid_t is 16 bits.
>
> I've just looked at the header.
> compat_pid_t is 32 bits.
> So Linux must have gained 32bit pids at some earlier time.
> (Historically Unix pids were 16 bit - even on 32bit systems.)
>
> Which makes the explicit pad in 'sparc' rather 'interesting'.
I saw it was there since the sparc kernel support got merged in
linux-1.3, possibly copied from an older sunos version.
> oh - compat_loff_t is only used in a couple of other places.
> neither care in any way about the alignment.
> (Provided get_user() doesn't fault on a 8n+4 aligned address.)
Ah right, I also see that after this series it's only used in to other
places: compat_resume_swap_area, which could also lose the
__packed annotation, and in the declaration of
compat_sys_sendfile64, where it makes no difference.
Arnd
