[PATCH] arm64: fix relocation of movz instruction with negative immediate

[Dave Martin]

On Mon, Jan 04, 2016 at 05:09:22PM +0100, Ard Biesheuvel wrote:
> The test whether a movz instruction with a signed immediate should be
> turned into a movn instruction (i.e., when the immediate is negative)
> is flawed, since the value of imm is always positive. So check sval
> instead.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/kernel/module.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
> index f4bc779e62e8..39e4a29cab50 100644
> --- a/arch/arm64/kernel/module.c
> +++ b/arch/arm64/kernel/module.c
> @@ -128,7 +128,7 @@ static int reloc_insn_movw(enum aarch64_reloc_op op, void *place, u64 val,

#define AARCH64_INSN_IMM_MOVNZ          AARCH64_INSN_IMM_MAX
#define AARCH64_INSN_IMM_MOVK           AARCH64_INSN_IMM_16

/* ... */

        if (imm_type == AARCH64_INSN_IMM_MOVNZ) {

/* ... */

>  		 * immediate is less than zero.
>  		 */
>  		insn &= ~(3 << 29);
> -		if ((s64)imm >= 0) {
> +		if (sval >= 0) {
>  			/* >=0: Set the instruction to MOVZ (opcode 10b). */
>  			insn |= 2 << 29;
>  		} else {

I _think_ this may be correct, but...

		}
		imm_type = AARCH64_INSN_IMM_MOVK;
	} 

        /* Update the instruction with the new encoding. */
        insn = aarch64_insn_encode_immediate(imm_type, insn, imm);

/* ... */

leaves imm_type as either AARCH64_INSN_IMM_16 or AARCH64_INSN_IMM_MOVK.

But because AARCH64_INSN_IMM_16 == AARCH64_INSN_IMM_MOVK (required for , the negative
overflow fudge is never applied, no?

        if (imm_type != AARCH64_INSN_IMM_16) {
                sval++;
                limit++;
        }


I'm wondering whether there is a less confusing way to do all this...

Cheers
---Dave