Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro - Yu, Yu-cheng

From: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
To: Dave Hansen <dave.hansen@intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
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	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	Pengfei Xu <pengfei.xu@intel.com>,
	Haitao Huang <haitao.huang@intel.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Subject: Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro
Date: Tue, 16 Mar 2021 10:44:46 -0700	[thread overview]
Message-ID: <1cbd8068-60e3-3fc9-9618-3262b179182d@intel.com> (raw)
In-Reply-To: <0c0b3663-3c01-c166-03fa-a3dbfb250da3@intel.com>

On 3/16/2021 10:28 AM, Dave Hansen wrote:
> 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?
> 

Yes, we can do two macros, probably in arch/x86/include/asm/vdso.h.