Subject: [PATCH 2/5] arm64: alternative: Allow immediate branch as alternative instruction

From: [email-masked] (Will Deacon)
Date: Thu, 26 Mar 2015 22:03:23 +0000

On Thu, Mar 19, 2015 at 01:59:33PM +0000, Marc Zyngier wrote:
> Since all immediate branches are PC-relative on Aarch64, these
> instructions cannot be used as an alternative with the simplistic
> approach we currently have (the immediate has been computed from
> the .altinstr_replacement section, and end-up being completely off
> if we insert it directly).
> 
> This patch handles the b and bl instructions in a different way,
> using the insn framework to recompute the immediate, and generate
> the right displacement.
> 
> Reviewed-by: Andre Przywara <[email-masked]>
> Signed-off-by: Marc Zyngier <[email-masked]>
> ---
>  arch/arm64/kernel/alternative.c | 55 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 53 insertions(+), 2 deletions(-)

[...]

>  static int __apply_alternatives(void *alt_region)
>  {
>  	struct alt_instr *alt;
> @@ -40,16 +83,24 @@ static int __apply_alternatives(void *alt_region)
>  	u8 *origptr, *replptr;
>  
>  	for (alt = region->begin; alt < region->end; alt++) {
> +		u32 insn;
> +		int i;
> +
>  		if (!cpus_have_cap(alt->cpufeature))
>  			continue;
>  
> -		BUG_ON(alt->alt_len > alt->orig_len);
> +		BUG_ON(alt->alt_len != alt->orig_len);
>  
>  		pr_info_once("patching kernel code\n");
>  
>  		origptr = (u8 *)&alt->orig_offset + alt->orig_offset;
>  		replptr = (u8 *)&alt->alt_offset + alt->alt_offset;
> -		memcpy(origptr, replptr, alt->alt_len);
> +
> +		for (i = 0; i < alt->alt_len; i += sizeof(insn)) {
> +			insn = get_alt_insn(origptr + i, replptr + i);
> +			*(u32 *)(origptr + i) = insn;

My brain's not firing on all cylinders right now, but do you need a
cpu_to_le32 here?

Will