linux-kernel - Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro

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Message-ID: <aff84067-5b9e-1335-e540-ef90ee133ac9@intel.com>
Date:   Tue, 16 Mar 2021 13:26:52 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro

On 3/16/2021 1:15 PM, Peter Zijlstra wrote:
> On Tue, Mar 16, 2021 at 01:05:30PM -0700, Yu, Yu-cheng wrote:
>> On 3/16/2021 12:57 PM, Peter Zijlstra wrote:
>>> On Tue, Mar 16, 2021 at 10:12:39AM -0700, Yu, Yu-cheng wrote:
>>>> Alternatively, there is another compiler-defined macro _CET_ENDBR that can
>>>> be used.  We can put the following in calling.h:
>>>>
>>>> #ifdef __CET__
>>>> #include <cet.h>
>>>> #else
>>>> #define _CET_ENDBR
>>>> #endif
>>>>
>>>> and then use _CET_ENDBR in other files.  How is that?
>>>>
>>>> In the future, in case we have kernel-mode IBT, ENDBR macros are also needed
>>>> for other assembly files.
>>>
>>> Can we please call it IBT_ENDBR or just ENDBR. CET is a horrible name,
>>> seeing how it is not specific.
>>>
>>
>> _CET_ENDBR is from the compiler and we cannot change it.  We can do:
>>
>> #define ENDBR _CET_ENDBR
>>
>> How is that?
> 
> Do we really want to include compiler headers? AFAICT it's not a
> built-in. Also what about clang?
> 
> This thing seems trivial enough to build our own, it's a single damn
> instruction. That also means we don't have to worry about changes to
> header files we don't control.
> 

Then, what about moving what I had earlier to vdso.h?
If we don't want __i386__ either, then make it two macros.

+.macro ENDBR
+#ifdef CONFIG_X86_CET
+#ifdef __i386__
+	endbr32
+#else
+	endbr64
+#endif
+#endif
+.endm