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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Tue, 1 Sep 2020 11:19:13 -0700
Message-ID: <CAMe9rOqhjhL=caGg=j4M=1+xFyEY-2vHsFn8vZNSaF33R5fVqQ@mail.gmail.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Florian Weimer <fweimer@redhat.com>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
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        Weijiang Yang <weijiang.yang@intel.com>
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On Tue, Sep 1, 2020 at 11:17 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Yu-cheng Yu:
>
> > On 9/1/2020 10:50 AM, Florian Weimer wrote:
> >> * Yu-cheng Yu:
> >>
> >>> Like other arch_prctl()'s, this parameter was 'unsigned long'
> >>> earlier. The idea was, since this arch_prctl is only implemented for
> >>> the 64-bit kernel, we wanted it to look as 64-bit only.  I will change
> >>> it back to 'unsigned long'.
> >> What about x32?  In general, long is rather problematic for x32.
> >
> > The problem is the size of 'long', right?
> > Because this parameter is passed in a register, and only the lower
> > bits are used, x32 works as well.
>
> The userspace calling convention leaves the upper 32-bit undefined.
> Therefore, this only works by accident if the kernel does not check that
> the upper 32-bit are zero, which is probably a kernel bug.
>
> It's unclear to me what you are trying to accomplish.  Why do you want
> to use unsigned long here?  The correct type appears to be unsigned int.
> This correctly expresses that the upper 32 bits of the register do not
> matter.
>

unsigned int is the correct type since only the lower 32 bits are used.

-- 
H.J.