linux-kernel - Re: [PATCH v11 25/25] x86/cet/shstk: Add arch

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Message-ID: <d78f0c7c-a672-2a74-974f-dffc67833abc@intel.com>
Date:   Tue, 1 Sep 2020 11:24:16 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     "H.J. Lu" <hjl.tools@...il.com>, Dave Martin <Dave.Martin@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

On 9/1/2020 11:17 AM, Florian Weimer 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.

Yes, you are right.  I will make it 'unsigned int'.

Thanks,
Yu-cheng