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

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Date:   Tue, 23 Aug 2016 13:32:02 +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 23 August 2016 at 11:16, Suzuki K Poulose <Suzuki.Poulose@....com> wrote:
> On 22/08/16 12:45, Ard Biesheuvel wrote:
>>
>> 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.
>
>
> We are modifying the alternative instruction to accommodate for the new PC,
> where this instruction will be executed from, while the referenced symbol
> remains the same. Hence the associated :lo12: doesn't change. Does that
> address your concern ? Or did I miss something ?
>

Ah, of course. Yes, that should work fine, given that the symbol stays
in place, and so its offset into the containing 4 KB page does not
change either.

Thanks,
Ard.