arm64: bti: Document behaviour for dynamically linked binaries

For dynamically linked binaries the interpreter is responsible for setting
PROT_BTI on everything except itself. The dynamic linker needs to be aware
of PROT_BTI, for example in order to avoid dropping that when marking
executable pages read only after doing relocations, and doing everything
in userspace ensures that we don't get any issues due to divergences in
behaviour between the kernel and dynamic linker within a single executable.
Add a comment indicating that this is intentional to the code to help
people trying to understand what's going on.
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/process.c

@@ -674,6 +674,11 @@ asmlinkage void __sched arm64_preempt_schedule_irq(void)
 int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
 			 bool has_interp, bool is_interp)
 {
+	/*
+	 * For dynamically linked executables the interpreter is
+	 * responsible for setting PROT_BTI on everything except
+	 * itself.
+	 */
 	if (is_interp != has_interp)
 		return prot;