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

[Dmitry Osipenko]

22.10.2020 19:23, Russell King - ARM Linux admin пишет:
> On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
>> On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
>> <linux@xxxxxxxxxxxxxxx> wrote:
>>>
>>> On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
>>>> On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@xxxxxxxxx> wrote:
>>>>>
>>>>> 22.10.2020 10:06, Ard Biesheuvel пишет:
>>>>>> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxx>
>>>>>>>>>> ---
>>>>>>>>>>  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.
>>>>
>>>> Russell, mind adding that? Or would you like me to update the patch in
>>>> the patch system?
>>>
>>> Rather than adding the IT, I'm suggesting that we solve it a different
>>> way - ensuring that the two bits of code are co-located. There's no
>>> reason for them to be separated, and the assembly code entry point is
>>> already called indirectly.
>>>
>>> The problem is the assembly ends up in the .text section which ends up
>>> at the start of the binary, but depending on the compiler, functions
>>> in .c files end up in their own sections. It would be good if, as
>>> Dmitry has shown that it is indeed possible, to have them co-located.
>>
>> Why is that better? I provided a minimal fix which has zero impact on
>> ARM builds, and minimal impact on Thumb2 builds, given that it retains
>> the exact same semantics as before, but using a different opcode.
> 
> I think you just described the reason there. Why should we force
> everything to use a different opcode when a short jump _should_
> suffice?
> 
> Your patch may be a single line, but it has a slightly greater
> impact than the alternative two line solution.
> 

But the two line change isn't portable to stable kernels as-is, isn't it?