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

Dear Michael,


Am 28.07.21 um 14:43 schrieb Michael Ellerman:

Paul Menzel [off-list ref] writes:

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Am 28.07.21 um 01:14 schrieb Benjamin Herrenschmidt:

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On Tue, 2021-07-27 at 10:45 +0200, Paul Menzel wrote:

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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