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

[Benjamin Herrenschmidt]

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.