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

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Date:   Mon, 2 Jul 2018 19:33:46 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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 Mon, Jul 2, 2018 at 7:30 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> ----- On Jul 2, 2018, at 10:18 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:
>
>> On Mon, Jul 2, 2018 at 7:01 PM Mathieu Desnoyers
>> <mathieu.desnoyers@...icios.com> wrote:
>>>
>>> One thing to consider is how we will implement the load of that pointer
>>> on the kernel side.
>>
>> Use "get_user()". It works for 64-bit objects too, and it will be
>> atomic in the 32-bit sub-parts on a 32-bit architecture.
>
> Is it really ? Last time we had this discussion, not all architectures
> guaranteed that reading a 64-bit integer would happen in two atomic
> 32-bit sub-parts. This was the main motivation for the LINUX_FIELD_u32_u64()
> macro as it stands today (rather than using a union).
>

If you're nervous, you could do this by open-coding:

#if BITS_PER_LONG == 64
get_user(...)
#else
get_user(...);
get_user(...);
#endif

No need to make the header more complicated just for this.