linux-kernel - Re: [PATCH 5/8] arm64: alternative: Add support for patching adrp instructions

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Date:   Mon, 22 Aug 2016 13:45:50 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH 5/8] arm64: alternative: Add support for patching adrp instructions

On 18 August 2016 at 15:10, Suzuki K Poulose <suzuki.poulose@....com> wrote:
> adrp uses PC-relative address offset to a page (of 4K size) of
> a symbol. If it appears in an alternative code patched in, we
> should adjust the offset to reflect the address where it will
> be run from. This patch adds support for fixing the offset
> for adrp instructions.
>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Andre Przywara <andre.przywara@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>  arch/arm64/kernel/alternative.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index d2ee1b2..71c6962 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -80,6 +80,19 @@ static u32 get_alt_insn(struct alt_instr *alt, u32 *insnptr, u32 *altinsnptr)
>                         offset = target - (unsigned long)insnptr;
>                         insn = aarch64_set_branch_offset(insn, offset);
>                 }
> +       } else if (aarch64_insn_is_adrp(insn)) {
> +               s32 orig_offset, new_offset;
> +               unsigned long target;
> +
> +               /*
> +                * If we're replacing an adrp instruction, which uses PC-relative
> +                * immediate addressing, adjust the offset to reflect the new
> +                * PC. adrp operates on 4K aligned addresses.
> +                */
> +               orig_offset  = aarch64_insn_adrp_get_offset(insn);
> +               target = ((unsigned long)altinsnptr & ~0xfffUL) + orig_offset;
> +               new_offset = target - ((unsigned long)insnptr & ~0xfffUL);
> +               insn = aarch64_insn_adrp_set_offset(insn, new_offset);

Are orig_offset and new_offset guaranteed to be equal modulo 4 KB?
Otherwise, you will have to track down and patch the associated :lo12:
add/ldr instruction as well.