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 20:28:08 +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 20:20, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> 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

Another note: this makes the badr unusable for switching from ARM to
Thumb2 mode. I didn't spot this right away because i have this patch

https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?h=arm-badr-cleanup&id=40ac917eef965aae3146e49f7948d1db9c01e968

in my working tree. I will send it out separately if we are ok with
going ahead with this

(That means we should retain the explicit .w suffix rather than using
the W() in this macro since the new code sequence is fundamentally
Thumb2 only)