linux-kernel - Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq

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Date:   Mon, 2 Jul 2018 19:22:22 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs

----- On Jul 2, 2018, at 7:16 PM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:

> ----- On Jul 2, 2018, at 7:06 PM, Linus Torvalds torvalds@...ux-foundation.org
> wrote:
> 
>> On Mon, Jul 2, 2018 at 4:00 PM Mathieu Desnoyers
>> <mathieu.desnoyers@...icios.com> wrote:
>>>
>>> Unfortunately, that rseq->rseq_cs field needs to be updated by user-space
>>> with single-copy atomicity. Therefore, we want 32-bit user-space to initialize
>>> the padding with 0, and only update the low bits with single-copy atomicity.
>> 
>> Well... It's actually still single-copy atomicity as a 64-bit value.
>> 
>> Why? Because it doesn't matter how you write the upper bits. You'll be
>> writing the same value to them (zero) anyway.
>> 
>> So who cares if the write ends up being two instructions, because the
>> write to the upper bits doesn't actually *do* anything.
>> 
>> Hmm?
> 
> Are there any kind of guarantees that a __u64 update on a 32-bit architecture
> won't be torn into something daft like byte-per-byte stores when performed
> from C code ?
> 
> I don't worry whether the upper bits get updated or how, but I really care
> about not having store tearing of the low bits update.

For the records, most updates of those low bits are done in assembly
from critical sections, for which we control exactly how the update is
performed.

However, there is one helper function in user-space that updates that value
from C through a volatile store, e.g.:

static inline void rseq_prepare_unload(void)
{
        __rseq_abi.rseq_cs = 0;
}

Thanks,

Mathieu

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

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