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Subject: Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO
 to generic C implementation.)
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Paul Menzel <pmenzel@molgen.mpg.de>, Michael Ellerman <mpe@ellerman.id.au>,
 Paul Mackerras <paulus@samba.org>, Christophe Leroy
 <christophe.leroy@csgroup.eu>
Date: Wed, 28 Jul 2021 09:14:48 +1000
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References: <a273c619-9258-e29a-24c3-ea47a13c4817@molgen.mpg.de>
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On Tue, 2021-07-27 at 10:45 +0200, Paul Menzel wrote:
> Dear Christophe,
> 
> 
> 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...

Cheers,
Ben.