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

[Paul Menzel]

Dear Michael,


Am 28.07.21 um 14:43 schrieb Michael Ellerman:
> Paul Menzel <pmenzel at 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.


Kind regards,

Paul