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

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Date:   Mon, 27 Sep 2021 21:01:59 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Richard Palethorpe <rpalethorpe@...e.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, rpalethorpe@...hiejp.com,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        LTP List <ltp@...ts.linux.it>
Subject: Re: [PATCH] x86/entry/ia32: Ensure s32 is sign extended to s64

On Mon, Sep 27, 2021 at 8:51 PM Florian Weimer <fweimer@...hat.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