Re: [PATCH 2/5] kernel/jump_label: implement generic support for relative references

[Peter Zijlstra]

On Wed, Jun 27, 2018 at 06:06:01PM +0200, Ard Biesheuvel wrote:
> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
> index 86ec0652d3b1..aa203dffe72c 100644
> --- a/include/linux/jump_label.h
> +++ b/include/linux/jump_label.h
> @@ -121,6 +121,32 @@ struct static_key {
>  #include <asm/jump_label.h>
>  
>  #ifndef __ASSEMBLY__
> +#ifdef CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE
> +
> +struct jump_entry {
> +	int code;
> +	int target;
> +	int key;
> +};

I much prefer you use 'u32' there.


> +static void jump_label_swap(void *a, void *b, int size)
> +{
> +	long delta = (unsigned long)a - (unsigned long)b;
> +	struct jump_entry *jea = a;
> +	struct jump_entry *jeb = b;
> +	struct jump_entry tmp = *jea;
> +
> +	jea->code	= jeb->code - delta;
> +	jea->target	= jeb->target - delta;
> +	jea->key	= jeb->key - delta;
> +
> +	jeb->code	= tmp.code + delta;
> +	jeb->target	= tmp.target + delta;
> +	jeb->key	= tmp.key + delta;
> +}
> +
>  static void
>  jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)
>  {
> @@ -56,7 +72,9 @@ jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)
>  
>  	size = (((unsigned long)stop - (unsigned long)start)
>  					/ sizeof(struct jump_entry));
> -	sort(start, size, sizeof(struct jump_entry), jump_label_cmp, NULL);
> +	sort(start, size, sizeof(struct jump_entry), jump_label_cmp,
> +	     IS_ENABLED(CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE) ? jump_label_swap
> +							      : NULL);
>  }

That will result in jump_label_swap being an unused symbol for some
compile options.

Would it not be much nicer to write that like:

static void jump_label_swap(void *a, void *b, int size)
{
	struct jump_entry *jea = a, *jeb = b;

#ifdef CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE
	long delta = a - b;

	jea->code += delta;
	jea->target += delta;
	jea->key += delta;

	jeb->code -= delta;
	jeb->target -= delta;
	jeb->key -= delta;
#else

	swap(*jea, *jeb);
}

And then unconditionally use jump_label_swap().