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

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Message-ID: <202010212028.32E8A5EF9B@keescook>
Date:   Wed, 21 Oct 2020 20:28:46 -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 03:00:06AM +0300, Dmitry Osipenko wrote:
> 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.

I might have just been distracted by the common "vfp" prefix. It's
possible that the text section shuffling just ended up being very large,
so probably this patch is right then!

-- 
Kees Cook