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

From: Michael Ellerman <[email-masked]>
Date: Mon, 02 Aug 2021 16:18:58 +1000

"Paul Murphy" <[email-masked]> writes:
>  
> (My apologies for however IBM's email client munges this)
>
>> 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.
>  
>  
> We have backports primed and ready for the next point release. They
> are waiting on the release manager to cherrypick them.

OK good, that is still the correct fix in the long run.

> I think we were aware that our VDSO usage may have exploited some
> peculiarities in how the ppc64 version was constructed (i.e hand
> written assembly which just didn't happen to clobber R30).

Yeah I was "somewhat surprised" that Go thought it could use r30 like
that across a VDSO call :D

But to be fair the ABI of the VDSO has always been a little fishy,
because the entry points pretend to be a transparent wrapper around
system calls, but then in a case like this are not.

> Go up to this point has only used the vdso function __kernel_clock_gettime; it
> is the only entry point which would need to explicitly avoid R30 for
> Go's sake.

I thought about limiting the workaround to just that code, but it seemed
silly and likely to come back to bite us. Once the compilers decides to
spill a non-volatile there are plenty of other ones to choose from.

cheers