linux-kernel - Re: [PATCH] ARM: decompressor: avoid ADRL pseudo-instruction

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Date:   Mon, 9 Nov 2020 21:53:18 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Jian Cai <jiancai@...gle.com>,
        Russell King <linux@...linux.org.uk>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Linus Walleij <linus.walleij@...aro.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: decompressor: avoid ADRL pseudo-instruction

On Mon, 9 Nov 2020 at 21:52, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> As Ard notes in
> commit 54781938ec34 ("crypto: arm/sha256-neon - avoid ADRL pseudo
> instruction")
> commit 0f5e8323777b ("crypto: arm/sha512-neon - avoid ADRL pseudo
> instruction")
>
>   The ADRL pseudo instruction is not an architectural construct, but a
>   convenience macro that was supported by the ARM proprietary assembler
>   and adopted by binutils GAS as well, but only when assembling in 32-bit
>   ARM mode. Therefore, it can only be used in assembler code that is known
>   to assemble in ARM mode only, but as it turns out, the Clang assembler
>   does not implement ADRL at all, and so it is better to get rid of it
>   entirely.
>
>   So replace the ADRL instruction with a ADR instruction that refers to
>   a nearer symbol, and apply the delta explicitly using an additional
>   instruction.
>
> We can use the same technique to generate the same offset. It looks like
> the ADRL pseudo instruction assembles to two SUB instructions in this
> case. Because the largest immediate operand that can be specified for
> this instruction is 0x400, and the distance between the reference and
> the symbol are larger than that, we need to use an intermediary symbol
> (cache_off in this case) to calculate the full range.
>
> Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> Suggested-by: Jian Cai <jiancai@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  arch/arm/boot/compressed/head.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index 2e04ec5b5446..b3eac6f9a709 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -1440,7 +1440,9 @@ ENTRY(efi_enter_kernel)
>                 mov     r4, r0                  @ preserve image base
>                 mov     r8, r1                  @ preserve DT pointer
>
> - ARM(          adrl    r0, call_cache_fn       )
> + ARM(          sub     r0, pc, #.L__efi_enter_kernel-cache_off )
> + ARM(          sub     r0, r0, #cache_off-call_cache_fn        )
> +.L__efi_enter_kernel:
>   THUMB(                adr     r0, call_cache_fn       )
>                 adr     r1, 0f                  @ clean the region of code we
>                 bl      cache_clean_flush       @ may run with the MMU off
> --
> 2.29.2.222.g5d2a92d10f8-goog
>

This is already fixed in Russell's for-next tree.