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

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Message-ID: <0c0b3663-3c01-c166-03fa-a3dbfb250da3@intel.com>
Date:   Tue, 16 Mar 2021 10:28:30 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...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>,
        Peter Zijlstra <peterz@...radead.org>,
        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>
Cc:     Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro

On 3/16/21 10:12 AM, Yu, Yu-cheng wrote:
> On 3/16/2021 8:49 AM, Dave Hansen wrote:
...
>> Is "#ifdef __i386__" the right thing to use here?  I guess ENDBR only
>> ends up getting used in the VDSO, but there's a lot of
>> non-userspace-exposed stuff in calling.h.  It seems a bit weird to have
>> the normally userspace-only __i386__ in there.
>>
>> I don't see any existing direct use of __i386__ in arch/x86/entry/vdso.
> 
> Good point.  My thought was, __i386__ comes from the compiler having the
> -m32 command-line option, and it is not dependent on anything else.
> 
> 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.

First of all, I think putting the macro in calling.h is wrong if it will
be used exclusively in the VDSO.  If it's VDSO-only, please put it in a
local header in the vdso/ directory, maybe even a new cet.h.

Also, Boris asked for two *different* macros for 32 and 64-bit:

https://lore.kernel.org/linux-api/20210310231731.GK23521@zn.tnic/

Could you do that in the next version, please?