Re: [PATCH] x86/entry/ia32: Ensure s32 is sign extended to s64

Arnd Bergmann
2021-09-27T19:02:20Z

On Mon, Sep 27, 2021 at 8:51 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Richard Palethorpe:
>
> > +#define __SC_COMPAT_CAST(t, a) \
> > + (__typeof(__builtin_choose_expr(__TYPE_IS_L(t), 0, 0U))) \
> > + (unsigned int)a
>
> So this casts to int (triggering sign extension) if the type on the
> 64-bit kernel side is long? But not in other cases (unsigned long,
> pointer)? Just double-checking.

Correct, this is the only case that is not already handled: anything smaller
than a 'long' is the same size on all architectures we support and we
ensure those are correctly sign- or zero-extended. 'unsigned long'
and any pointer are zero-extended by the entry code from 32-bit user
space to a 64-bit register in the kernel. Only signed 'long' requires
explicit sign-extending from the userspace 'long' to the kernel function
argument.

Arnd