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

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Date:   Tue, 16 Mar 2021 14:01:08 -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 1:26 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 */