Re: [PATCH v4 2/4] net: socket: rework SIOC?IFMAP ioctls

[Arnd Bergmann <arnd@kernel.org>]

On Tue, Nov 24, 2020 at 5:13 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Arnd Bergmann
> > Sent: 24 November 2020 15:18
> >
> > SIOCGIFMAP and SIOCSIFMAP currently require compat_alloc_user_space()
> > and copy_in_user() for compat mode.
> >
> > Move the compat handling into the location where the structures are
> > actually used, to avoid using those interfaces and get a clearer
> > implementation.
> >
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> > changes in v3:
> >  - complete rewrite
> ...
> >  include/linux/compat.h | 18 ++++++------
> >  net/core/dev_ioctl.c   | 64 +++++++++++++++++++++++++++++++++---------
> >  net/socket.c           | 39 ++-----------------------
> >  3 files changed, 62 insertions(+), 59 deletions(-)
> >
> > diff --git a/include/linux/compat.h b/include/linux/compat.h
> > index 08dbd34bb7a5..47496c5eb5eb 100644
> > --- a/include/linux/compat.h
> > +++ b/include/linux/compat.h
> > @@ -96,6 +96,15 @@ struct compat_iovec {
> >       compat_size_t   iov_len;
> >  };
> >
> > +struct compat_ifmap {
> > +     compat_ulong_t mem_start;
> > +     compat_ulong_t mem_end;
> > +     unsigned short base_addr;
> > +     unsigned char irq;
> > +     unsigned char dma;
> > +     unsigned char port;
> > +};
>
> Isn't the only difference the number of pad bytes at the end?

No, the main difference is in the first two fields, which are
'unsigned long' and therefore different. The three-byte padding
is in fact the same on all architectures (including x86) that
have a compat mode, though it might be different on
m68k and arm-oabi, which have slightly special struct
alignment rules.

It could be done with two assignments and a memcpy, but
I like the individual assignments better here.

      Arnd