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

[Michael Ellerman <mpe@ellerman.id.au>]

Paul Menzel <pmenzel@molgen.mpg.de> writes:

> Dear Michael,
>
>
> 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?

cheers


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
+# 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