x86/linkage: Add SYM_PIC_ALIAS() macro helper to emit symbol aliases

Startup code that may execute from the early 1:1 mapping of memory will
be confined into its own address space, and only be permitted to access
ordinary kernel symbols if this is known to be safe.

Introduce a macro helper SYM_PIC_ALIAS() that emits a __pi_ prefixed
alias for a symbol, which allows startup code to access it.

Signed-off-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Dionna Amalie Glaze <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Kevin Loughlin <[e-mail]>
Cc: Len Brown <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Rafael J. Wysocki <[e-mail]>
Cc: Tom Lendacky <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/linkage.h

···
 #define SYM_FUNC_START_WEAK_NOALIGN(name)		\
 	SYM_START(name, SYM_L_WEAK, SYM_A_NONE)
 
+/*
+ * Expose 'sym' to the startup code in arch/x86/boot/startup/, by emitting an
+ * alias prefixed with __pi_
+ */
+#ifdef __ASSEMBLER__
+#define SYM_PIC_ALIAS(sym)	SYM_ALIAS(__pi_ ## sym, sym, SYM_L_GLOBAL)
+#else
+#define SYM_PIC_ALIAS(sym)	extern typeof(sym) __PASTE(__pi_, sym) __alias(sym)
+#endif
+
 #endif /* _ASM_X86_LINKAGE_H */