linux-kernel - Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

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Date:   Thu, 22 Oct 2020 03:00:06 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Russell King <linux@...linux.org.uk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel
 compilation error

22.10.2020 02:40, Kees Cook пишет:
> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
>> The vfp_kmode_exception() function now is unreachable using relative
>> branching in THUMB2 kernel configuration, resulting in a "relocation
>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
>> linker error. Let's use long jump in order to fix the issue.
> 
> Eek. Is this with gcc or clang?

GCC 9.3.0

>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> 
> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> That commit may have implicitly moved the location of .vfp11_veneer,
> though I thought I had chosen the correct position.

I re-checked that the fixes tag is correct.

>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>> ---
>>  arch/arm/vfp/vfphw.S | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
>> index 4fcff9f59947..6e2b29f0c48d 100644
>> --- a/arch/arm/vfp/vfphw.S
>> +++ b/arch/arm/vfp/vfphw.S
>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
>>  	ldr	r3, [sp, #S_PSR]	@ Neither lazy restore nor FP exceptions
>>  	and	r3, r3, #MODE_MASK	@ are supported in kernel mode
>>  	teq	r3, #USR_MODE
>> -	bne	vfp_kmode_exception	@ Returns through lr
>> +	ldr	r1, =vfp_kmode_exception
>> +	bxne	r1			@ Returns through lr
>>  
>>  	VFPFMRX	r1, FPEXC		@ Is the VFP enabled?
>>  	DBGSTR1	"fpexc %08x", r1
> 
> This seems like a workaround though? I suspect the vfp11_veneer needs
> moving?
> 

I don't know where it needs to be moved. Please feel free to make a
patch if you have a better idea, I'll be glad to test it.