start_kernel: Add __no_stack_protector function attribute

Back during the discussion of
commit a9a3ed1eff ("x86: Fix early boot crash on gcc-10, third try")
we discussed the need for a function attribute to control the omission
of stack protectors on a per-function basis; at the time Clang had
support for no_stack_protector but GCC did not. This was fixed in
gcc-11. Now that the function attribute is available, let's start using
it.

Callers of boot_init_stack_canary need to use this function attribute
unless they're compiled with -fno-stack-protector, otherwise the canary
stored in the stack slot of the caller will differ upon the call to
boot_init_stack_canary. This will lead to a call to __stack_chk_fail()
then panic.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94722
Link: https://lore.kernel.org/all/20200316130414.GC12561@hirez.programming.kicks-ass.net/
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20230412-no_stackp-v2-1-116f9fe4bbe7@google.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

Signed-off-by: ndesaulniers@google.com <ndesaulniers@google.com>

arch/powerpc/kernel/smp.c

@@ -1605,6 +1605,7 @@ static void add_cpu_to_masks(int cpu)
 }
 
 /* Activate a secondary processor. */
+__no_stack_protector
 void start_secondary(void *unused)
 {
 	unsigned int cpu = raw_smp_processor_id();

include/linux/compiler_attributes.h

@@ -255,6 +255,18 @@
  */
 #define __noreturn                      __attribute__((__noreturn__))
 
+/*
+ * Optional: only supported since GCC >= 11.1, clang >= 7.0.
+ *
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-no_005fstack_005fprotector-function-attribute
+ *   clang: https://clang.llvm.org/docs/AttributeReference.html#no-stack-protector-safebuffers
+ */
+#if __has_attribute(__no_stack_protector__)
+# define __no_stack_protector		__attribute__((__no_stack_protector__))
+#else
+# define __no_stack_protector
+#endif
+
 /*
  * Optional: not supported by gcc.
  *

init/main.c

@@ -877,7 +877,8 @@ static void __init print_unknown_bootoptions(void)
 	memblock_free(unknown_options, len);
 }
 
-asmlinkage __visible void __init __no_sanitize_address __noreturn start_kernel(void)
+asmlinkage __visible __init __no_sanitize_address __noreturn __no_stack_protector
+void start_kernel(void)
 {
 	char *command_line;
 	char *after_dashes;