arm64: alternatives: make alt_region const

We never alter a struct alt_region after creation, and we open-code the
bounds of the kernel alternatives region in two functions. The
duplication is a bit unfortunate for clarity (and in future we're likely
to have more functions altering alternative regions), and to avoid
accidents it would be good to make the structure const.

This patch adds a shared struct `kernel_alternatives` alt_region for the
main kernel image, and marks the alt_regions as const to prevent
unintentional modification.

There should be no functional change as a result of this patch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220912162210.3626215-6-mark.rutland@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/alternative.c

@@ -133,8 +133,9 @@ static void clean_dcache_range_nopatch(u64 start, u64 end)
 	} while (cur += d_size, cur < end);
 }
 
-static void __nocfi __apply_alternatives(struct alt_region *region, bool is_module,
-				 unsigned long *feature_mask)
+static void __nocfi __apply_alternatives(const struct alt_region *region,
+					 bool is_module,
+					 unsigned long *feature_mask)
 {
 	struct alt_instr *alt;
 	__le32 *origptr, *updptr;
@@ -190,17 +191,17 @@ static void __nocfi __apply_alternatives(struct alt_region *region, bool is_modu
 	}
 }
 
+static const struct alt_region kernel_alternatives = {
+	.begin	= (struct alt_instr *)__alt_instructions,
+	.end	= (struct alt_instr *)__alt_instructions_end,
+};
+
 /*
  * We might be patching the stop_machine state machine, so implement a
  * really simple polling protocol here.
  */
 static int __apply_alternatives_multi_stop(void *unused)
 {
-	struct alt_region region = {
-		.begin	= (struct alt_instr *)__alt_instructions,
-		.end	= (struct alt_instr *)__alt_instructions_end,
-	};
-
 	/* We always have a CPU 0 at this point (__init) */
 	if (smp_processor_id()) {
 		while (!all_alternatives_applied)
@@ -213,7 +214,8 @@ static int __apply_alternatives_multi_stop(void *unused)
 				  ARM64_NPATCHABLE);
 
 		BUG_ON(all_alternatives_applied);
-		__apply_alternatives(&region, false, remaining_capabilities);
+		__apply_alternatives(&kernel_alternatives, false,
+				     remaining_capabilities);
 		/* Barriers provided by the cache flushing */
 		all_alternatives_applied = 1;
 	}
@@ -236,17 +238,13 @@ void __init apply_alternatives_all(void)
  */
 void __init apply_boot_alternatives(void)
 {
-	struct alt_region region = {
-		.begin	= (struct alt_instr *)__alt_instructions,
-		.end	= (struct alt_instr *)__alt_instructions_end,
-	};
-
 	/* If called on non-boot cpu things could go wrong */
 	WARN_ON(smp_processor_id() != 0);
 
 	pr_info("applying boot alternatives\n");
 
-	__apply_alternatives(&region, false, &boot_capabilities[0]);
+	__apply_alternatives(&kernel_alternatives, false,
+			     &boot_capabilities[0]);
 }
 
 #ifdef CONFIG_MODULES