linux-kernel - Re: [RFC PATCH for 5.8 3/4] rseq: Introduce RSEQ_FLAG_RELIABLE_CPU

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Date:   Wed, 8 Jul 2020 11:33:51 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fw@...eb.enyo.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     carlos <carlos@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Neel Natu <neelnatu@...gle.com>
Subject: Re: [RFC PATCH for 5.8 3/4] rseq: Introduce
 RSEQ_FLAG_RELIABLE_CPU_ID

[ Context for Linus: I am dropping this RFC patch, but am curious to
  hear your point of view on exposing to user-space which system call
  behavior fixes are present in the kernel, either through feature
  flags or system-call versioning. The intent is to allow user-space
  to make better decisions on whether it should use a system call or
  rely on fallback behavior. ]

----- On Jul 7, 2020, at 3:55 PM, Florian Weimer fw@...eb.enyo.de wrote:

> * Carlos O'Donell:
> 
>> It's not a great fit IMO. Just let the kernel version be the arbiter of
>> correctness.
> 
> For manual review, sure.  But checking it programmatically does not
> yield good results due to backports.  Even those who use the stable
> kernel series sometimes pick up critical fixes beforehand, so it's not
> reliable possible for a program to say, “I do not want to run on this
> kernel because it has a bad version”.  We had a recent episode of this
> with the Go runtime, which tried to do exactly this.

FWIW, the kernel fix backport issue would also be a concern if we exposed
a numeric "fix level version" with specific system calls: what should
we do if a distribution chooses to include one fix in the sequence,
but not others ? Identifying fixes are "feature flags" allow
cherry-picking specific fixes in a backport, but versions would not
allow that.

That being said, maybe it's not such a bad thing to _require_ the
entire series of fixes to be picked in backports, which would be a
fortunate side-effect of the per-syscall-fix-version approach.

But I'm under the impression that such a scheme ends up versioning
a system call, which I suspect will be a no-go from Linus' perspective.

Thanks,

Mathieu

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