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Date:   Mon, 6 Jul 2020 14:02:19 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
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Subject: Re: [PATCH 2/3] Linux: Use rseq in sched_getcpu if available (v9)
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----- On Jul 6, 2020, at 1:50 PM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>> Now we need to discuss how we introduce that fix in a way that will
>> allow user-space to trust the __rseq_abi.cpu_id field's content.
> 
> I don't think that's necessary.  We can mention it in the glibc
> distribution notes on the wiki.
> 
>> The usual approach to kernel bug fixing is typically to push the fix,
>> mark it for stable kernels, and expect everyone to pick up the
>> fixes. I wonder how comfortable glibc would be to replace its
>> sched_getcpu implementation with a broken-until-fixed kernel rseq
>> implementation without any mechanism in place to know whether it can
>> trust the value of the cpu_id field. I am extremely reluctant to do
>> so.
> 
> We have already had similar regressions in sched_getcpu, and we didn't
> put anything into glibc to deal with those.

Was that acceptable because having a wrong cpu number would never trigger
corruption, only slowdowns ?

In the case of rseq, having the wrong cpu_id value is a real issue which
will lead to corruption and crashes. So I maintain my reluctance to introduce
the fix without any way for userspace to know whether the cpu_id field
value is reliable.

What were the reasons why it was OK to have this kind of regression in
sched_getcpu in the past, and are they still valid in the context of
rseq ?

Thanks,

Mathieu

> 
> Just queue the fix for the stable kernels.  I expect that all
> distributions track stable kernel branches in some way, so just put into
> the kernel commit message that this commit is needed for a working
> sched_getcpu in glibc 2.32 and later.
> 
> Once the upstream fix is in Linus' tree, I'm going to file a request to
> backport the fix into the Red Hat Enterprise Linux 8.
> 
> Thanks for finding the root cause so quickly,
> Florian

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com