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

From: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	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>,
	Jarkko Sakkinen <jarkko@kernel.org>
Subject: Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro
Date: Tue, 16 Mar 2021 14:01:08 -0700	[thread overview]
Message-ID: <9da6397f-326b-7891-5810-a5ac9e0def5d@intel.com> (raw)
In-Reply-To: <aff84067-5b9e-1335-e540-ef90ee133ac9@intel.com>

On 3/16/2021 1:26 PM, Yu, Yu-cheng wrote:
> 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

I will make it like the following:

diff --git a/arch/x86/include/asm/vdso.h b/arch/x86/include/asm/vdso.h
index 98aa103eb4ab..4c0262dcb93d 100644
--- a/arch/x86/include/asm/vdso.h
+++ b/arch/x86/include/asm/vdso.h
@@ -52,6 +52,15 @@ extern int map_vdso_once(const struct vdso_image 
*image, unsigned long addr);
  extern bool fixup_vdso_exception(struct pt_regs *regs, int trapnr,
  				 unsigned long error_code,
  				 unsigned long fault_addr);
-#endif /* __ASSEMBLER__ */
+#else /* __ASSEMBLER__ */
+
+#ifdef CONFIG_X86_CET
+#define ENDBR64 endbr64
+#define ENDBR32 endbr32
+#else /*!CONFIG_X86_CET */
+#define ENDBR64
+#define ENDBR32
+#endif

+#endif /* __ASSEMBLER__ */
  #endif /* _ASM_X86_VDSO_H */
