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From: Michael Ellerman <mpe@ellerman.id.au>
To: Paul Menzel <pmenzel@molgen.mpg.de>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Derek Parker <parkerderek86@gmail.com>,
	Dmitrii Okunev <xaionaro@gmail.com>,
	murp@ibm.com, linuxppc-dev@lists.ozlabs.org,
	laboger@linux.vnet.ibm.com
Subject: Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)
Date: Wed, 28 Jul 2021 22:43:36 +1000	[thread overview]
Message-ID: <878s1q1udj.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <4f037af0-5066-ebb9-53a6-733b3bd8eeac@molgen.mpg.de>

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'll see if we can work around it in the kernel. Are you able to test a
kernel patch if I send you one?

cheers

  reply	other threads:[~2021-07-28 12:44 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-27  8:45 Paul Menzel
2021-07-27 23:14 ` Benjamin Herrenschmidt
2021-07-28  8:26   ` Paul Menzel
2021-07-28 12:43     ` Michael Ellerman [this message]
2021-07-28 12:53       ` Paul Menzel
2021-07-29  7:41         ` Michael Ellerman
2021-07-29  8:33           ` Paul Menzel
2021-07-29 12:58             ` Michael Ellerman
     [not found]         ` <875ywt1s9r.fsf__45665.8238823124$1627544516$gmane$org@mpe.ellerman.id.au>
2021-07-29  8:48           ` Andreas Schwab
2021-08-02  6:04             ` Michael Ellerman
     [not found]       ` <OF44F7146F.67A4C1C2-ON00258720.004DBF64-00258720.004FCFCC@ibm.com>
2021-08-02  6:18         ` Michael Ellerman
2021-08-02 12:48           ` Benjamin Herrenschmidt
2021-08-02 18:07           ` Segher Boessenkool
2021-08-02 18:14           ` Lynn Boger

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