Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)

[Paul Menzel <pmenzel@molgen.mpg.de>]

Dear Michael,


Am 29.07.21 um 09:41 schrieb Michael Ellerman:
> Paul Menzel <pmenzel@molgen.mpg.de> writes:

>> Am 28.07.21 um 14:43 schrieb Michael Ellerman:
>>> Paul Menzel <pmenzel@molgen.mpg.de> writes:
>>>> Am 28.07.21 um 01:14 schrieb Benjamin Herrenschmidt:
>>>>> On Tue, 2021-07-27 at 10:45 +0200, Paul Menzel wrote:
>>>>
>>>>>> On ppc64le Go 1.16.2 from Ubuntu 21.04 terminates with a segmentation
>>>>>> fault [1], and it might be related to *[release-branch.go1.16] runtime:
>>>>>> fix crash during VDSO calls on PowerPC* [2], conjecturing that commit
>>>>>> ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)
>>>>>> added in Linux 5.11 causes this.
>>>>>>
>>>>>> If this is indeed the case, this would be a regression in userspace. Is
>>>>>> there a generic fix or should the change be reverted?
>>>>>
>>>>>   From the look at the links you posted, this appears to be completely
>>>>> broken assumptions by Go that some registers don't change while calling
>>>>> what essentially are external library functions *while inside those
>>>>> functions* (ie in this case from a signal handler).
>>>>>
>>>>> I suppose it would be possible to build the VDSO with gcc arguments to
>>>>> make it not use r30, but that's just gross...
>>>>
>>>> Thank you for looking into this. No idea, if it falls under Linux’ no
>>>> regression policy or not.
>>>
>>> Reluctantly yes, I think it does. Though it would have been good if it
>>> had been reported to us sooner.
>>>
>>> It looks like that Go fix is only committed to master, and neither of
>>> the latest Go 1.16 or 1.15 releases contain the fix? ie. there's no way
>>> for a user to get a working version of Go other than building master?
>>
>> I heard it is going to be in Go 1.16.7, but I do not know much about Go.
>> Maybe the folks in Cc can chime in.
>>
>>> I'll see if we can work around it in the kernel. Are you able to test a
>>> kernel patch if I send you one?
>>
>> Yes, I could test a Linux kernel patch on ppc64le (POWER 8) running
>> Ubuntu 21.04.
> 
> Thanks, would be great if you can test on your setup. Patch below.
> 
> I haven't been able to reproduce the crash by following the instructions
> in your bug report, I have go1.13.8, I guess the crash is only in newer
> versions?

I only used go version 1.16.2 packaged in Ubuntu 21.04 (1.16~0ubuntu1).

> diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
> index 2813e3f98db6..3c5baaa6f1e7 100644
> --- a/arch/powerpc/kernel/vdso64/Makefile
> +++ b/arch/powerpc/kernel/vdso64/Makefile
> @@ -27,6 +27,13 @@ KASAN_SANITIZE := n
>   
>   ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
>   	-Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both
> +
> +# Go prior to 1.16.x assumes r30 is not clobbered by any VDSO code. That used to be true

Probably that needs to be 1.16.7.

> +# by accident when the VDSO was hand-written asm code, but may not be now that the VDSO is
> +# compiler generated. To avoid breaking Go tell GCC not to use r30. Impact on code
> +# generation is minimal, it will just use r29 instead.
> +ccflags-y += $(call cc-option, -ffixed-r30)
> +
>   asflags-y := -D__VDSO64__ -s
>   
>   targets += vdso64.lds

With this applied to Linux, go does not crash with a segmentation fault 
anymore.

Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>

(Probably the commit should be tagged for the stable series too.)


Kind regards,

Paul