linux-kernel - Re: [RFC PATCH] ARM: Use logical or instead of addition for badr address calculation

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Date:   Tue, 19 Jun 2018 20:20:02 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Russell King <linux@...linux.org.uk>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] ARM: Use logical or instead of addition for badr
 address calculation

On 19 June 2018 at 20:17, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> On 19 June 2018 at 19:24, Guenter Roeck <linux@...ck-us.net> wrote:
>> On Tue, Jun 19, 2018 at 03:35:07PM +0200, Ard Biesheuvel wrote:
>>> >>
>>> >> +       /* this needs to be a separate macro or \@ does not work correctly
>>> >> */
>>> >> +       .macro  __badr, c, rd, sym
>>> >> +       .eqv    .Lsym\@, \sym
>>> >> +       adr\c   \rd, .Lsym\@ + 1
>>> >
>>> >
>>> > Wild shot, but the following works for me.
>>> >
>>> >         .eqv    .Lsym\@, \sym + 1
>>> >         adr\c   \rd, .Lsym\@
>>> >
>>> > Does it make sense ?
>>> >
>>>
>>> Interesting. Do you mean this works with your 2.30 binutils that
>>> triggers the original issue?
>>>
>>
>> Yes, it does. It is also the solution used for some graphics libraries,
>> though of course now I don't find the link anymore.
>>
>> Guess this is going nowhere given Russell's response, so I'll just
>> live with it, like obviously everyone else does already. I built
>> a toolchain using gcc 7.3.0 and binutils 2.28.1 which "solves"
>> the problem for me.
>>
>
> If we can live with using a wide encoding unconditionally, we could
> consider something like below. That forces the symbol references to be
> resolved at link time, which means we completely sidestep the new GAS
> code.
>
> -----------8<----------------
> From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Date: Tue, 16 Jan 2018 12:12:45 +0000
> Subject: [PATCH] ARM: assembler: prevent ADR from setting the Thumb bit twice
>
> To work around recent issues where ADR references to Thumb function
> symbols may or may not have the Thumb bit set already when they are
> resolved by GAS, reference the symbol indirectly via a global symbol
> typed as 'function', and emit a relocation that lets the linker fix
> up the reference.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index 6ae42ad29518..1a55ce4b245c 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -195,13 +195,22 @@
>         .irp    c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo
>         .macro  badr\c, rd, sym
>  #ifdef CONFIG_THUMB2_KERNEL
> -       adr\c   \rd, \sym + 1
> +       __badr  \c, \rd, \sym
>  #else
>         adr\c   \rd, \sym
>  #endif
>         .endm
>         .endr
>
> +       /* this needs to be a separate macro or \@ does not work correctly */
> +       .macro          __badr, c, rd, sym
> +       .globl          .Lsym_\sym\()_\@
> +       .type           .Lsym_\sym\()_\@, %function
> +       .set            .Lsym_\sym\()_\@, \sym
> +       .reloc          ., R_ARM_THM_ALU_PREL_11_0, .Lsym_\sym\()_\@
> +       adr\c\().w      \rd, .
> +       .endm
> +
>  /*
>   * Get current thread_info.
>   */

It seems we can drop the .globl btw