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

[Guenter Roeck <linux@roeck-us.net>]

Hi Ard,

On 06/19/2018 12:48 AM, Ard Biesheuvel wrote:
> On 19 June 2018 at 07:07, Guenter Roeck <linux@roeck-us.net> wrote:
>> Modern assemblers may take the ISA into account when resolving local
>> symbols. This can result in bad address calculations when using badr
>> in the wrong location since the offset + 1 may be added twice, resulting
>> in an even address target for THUMB instructions. This in turn results
>> in an exception at (destination address + 2).
>>
>> Unhandled exception: IPSR = 00000006 LR = fffffff1
>> CPU: 0 PID: 1 Comm: init Not tainted 4.18.0-rc1-00026-gf773e5bdf0c9 #15
>> Hardware name: MPS2 (Device Tree Support)
>> PC is at ret_fast_syscall+0x2/0x58
>> LR is at tty_ioctl+0x2a5/0x528
>> pc : [<21009002>]    lr : [<210d1535>]    psr: 4000000b
>> sp : 21825fa8  ip : 0000001c  fp : 21a95892
>> r10: 00000000  r9 : 21824000  r8 : 210091c0
>> r7 : 00000036  r6 : 21ae1ee0  r5 : 00000000  r4 : 21ae1f3c
>> r3 : 00000000  r2 : 3d9adc25  r1 : 00000000  r0 : 00000000
>> xPSR: 4000000b
>> CPU: 0 PID: 1 Comm: init Not tainted 4.18.0-rc1-00026-gf773e5bdf0c9 #15
>> Hardware name: MPS2 (Device Tree Support)
>> [<2100bd8d>] (unwind_backtrace) from [<2100b13b>] (show_stack+0xb/0xc)
>> [<2100b13b>] (show_stack) from [<2100b87b>] (__invalid_entry+0x4b/0x4c)
>>
>> Fix the problem by using a logical or instead of an addition. This is
>> less efficient but guaranteed to work.
>>
>> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
>> ---
>> RFC: I don't really like this, but my ARM assembler knowledge is quite
>> limited. Just dropping the "+ 1" from badr doesn't work because some
>> other code needs it (the image hangs completely if I try that).
>> Ultimately I don't even know if the invoke_syscall macro should just
>> have used adr instead of badr (but then how did this ever work ?).
>>
>> Seen with various toolchains based on gcc 7.x and binutils 2.30 when
>> building and testing MPS2 images.
>>
> 
> Hello Guenter,
> 
> This issue has been discussed before. It appears the binutils people
> suddenly started caring about the thumb annotation of local function
> symbols, resulting in behavior that is not backwards compatible.
> 
> https://marc.info/?l=linux-arm-kernel&m=151143776213636&w=2
> https://sourceware.org/bugzilla/show_bug.cgi?id=21458
> 
> Can you try the fix below please?
> 
>>   arch/arm/include/asm/assembler.h | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
>> index 0cd4dccbae78..24c87ff2060f 100644
>> --- a/arch/arm/include/asm/assembler.h
>> +++ b/arch/arm/include/asm/assembler.h
>> @@ -195,7 +195,8 @@
>>          .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
>> +       adr\c   \rd, \sym
>> +       orr     \rd, #1
>>   #else
>>          adr\c   \rd, \sym
>>   #endif
>> --
>> 2.7.4
> 
> ----------8<------------
> From: Ard Biesheuvel <ard.biesheuvel@linaro.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 local symbol
> typed as 'object', stripping the ARM/Thumb annotation.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> 
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index 6ae42ad29518..dd2ff94ad90b 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -195,13 +195,19 @@
>          .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
> +       .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 ?

Thanks,
Guenter