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 Wed, 28 Jul 2021 10:26:29 +0200 (CEST)
Subject: Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to
 generic C implementation.)
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Michael Ellerman <mpe@ellerman.id.au>, Paul Mackerras <paulus@samba.org>,
 Christophe Leroy <christophe.leroy@csgroup.eu>
References: <a273c619-9258-e29a-24c3-ea47a13c4817@molgen.mpg.de>
 <3661999754da1a5e5c810fa669654cc7db95b059.camel@kernel.crashing.org>
From: Paul Menzel <pmenzel@molgen.mpg.de>
Message-ID: <4f037af0-5066-ebb9-53a6-733b3bd8eeac@molgen.mpg.de>
Date: Wed, 28 Jul 2021 10:26:29 +0200
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Dear Benjamin,


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.


Kind regards,

Paul