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 18:57:03 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Ard Biesheuvel <ardb@...nel.org>, Kees Cook <keescook@...omium.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel
 compilation error

22.10.2020 10:06, Ard Biesheuvel пишет:
> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@...omium.org> wrote:
>>
>> 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!
>>
> 
> I already sent a fix for this issue:
> 
> https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> 

The offending commit contains stable tag, so I assume that fixes tag is
mandatory. Yours patch misses the fixes tag.