cfi: Add CFI_NOSEAL()

Add a CFI_NOSEAL() helper to mark functions that need to retain their
CFI information, despite not otherwise leaking their address.

Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

arch/x86/include/asm/cfi.h

···
  * Copyright (C) 2022 Google LLC
  */
 #include <linux/bug.h>
+#include <asm/ibt.h>
 
 /*
  * An overview of the various calling conventions...
···
 	return 0;
 }
 #endif /* CONFIG_CFI_CLANG */
+
+#if HAS_KERNEL_IBT == 1
+#define CFI_NOSEAL(x)	asm(IBT_NOSEAL(__stringify(x)))
+#endif
 
 #endif /* _ASM_X86_CFI_H */

include/linux/cfi.h

···
 #endif /* CONFIG_ARCH_USES_CFI_TRAPS */
 #endif /* CONFIG_MODULES */
 
+#ifndef CFI_NOSEAL
+#define CFI_NOSEAL(x)
+#endif
+
 #endif /* _LINUX_CFI_H */