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

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Message-ID: <YFER79kU+ukn3YZr@hirez.programming.kicks-ass.net>
Date:   Tue, 16 Mar 2021 21:15:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
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 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.