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

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Message-ID: <202010211637.7CFD8435@keescook>
Date:   Wed, 21 Oct 2020 16:40:34 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dmitry Osipenko <digetx@...il.com>
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

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?

> 
> 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.

> 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?

-- 
Kees Cook