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

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Date:   Mon, 22 Aug 2016 12:19:22 +0100
From:   Will Deacon <will.deacon@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, mark.rutland@....com,
        andre.przywara@....com, Marc Zyngier <marc.zyngier@....com>
Subject: Re: [PATCH 5/8] arm64: alternative: Add support for patching adrp
 instructions

On Thu, Aug 18, 2016 at 02:10:29PM +0100, Suzuki K Poulose 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);

The masking with ~0xfffUL might be nicer if you write it as
align_down(ptr, SZ_4K);

> +		insn = aarch64_insn_adrp_set_offset(insn, new_offset);
>  	}
>  
>  	return insn;

I wonder if we shouldn't have a catch-all for any instructions performing
PC-relative operations here, because silent corruption of the instruction
stream is pretty horrible. What other instructions are there? ADR, LDR
(literal), ... ?

Will