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

[Yu, Yu-cheng]

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 */