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

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Date:   Tue, 3 Jul 2018 14:41:01 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <andi@...stfloor.org>,
        heiko carstens <heiko.carstens@...ibm.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.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>, 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>,
        Michal Simek <michal.simek@...inx.com>,
        schwidefsky <schwidefsky@...ibm.com>, gor <gor@...ux.ibm.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs

----- On Jul 3, 2018, at 2:28 PM, Peter Zijlstra peterz@...radead.org wrote:

> On Tue, Jul 03, 2018 at 02:15:34PM -0400, Mathieu Desnoyers wrote:
>> ----- On Jul 3, 2018, at 2:11 PM, Peter Zijlstra peterz@...radead.org wrote:
>> 
>> > On Tue, Jul 03, 2018 at 01:58:37PM -0400, Mathieu Desnoyers wrote:
>> >> I can modify the ABI to put the cpu_id_start and cpu_id fields inside
>> >> a union, and update it with a single store.
>> >> 
>> >> Thoughts ?
>> > 
>> > Let's keep them for now, we can always frob this later, they are aligned
>> > and proper, no need to expose that union to userspace.
>> 
>> Isn't it weird to change the API of an exposed public uapi header ?
> 
> Sure, just keep it as is. We don't need an exposed union to do a single
> store there.
> 
> Something like the ugly below preserves API but still does a single
> store.
> 
> But sure, if you want to expose that union for some reason, then now is
> the time.

User-space won't ever want to read cpu_id_start and cpu_id from a single
u64 load, it serves no purpose to do so. So I'm OK with keeping those as
is and defining a local union for the __put_user() update.

Thanks!

Mathieu

> 
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 22b6acf1ad63..e956c48b5f83 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -85,10 +85,17 @@ static int rseq_update_cpu_id(struct task_struct *t)
> {
> 	u32 cpu_id = raw_smp_processor_id();
> 
> -	if (__put_user(cpu_id, &t->rseq->cpu_id_start))
> -		return -EFAULT;
> -	if (__put_user(cpu_id, &t->rseq->cpu_id))
> +	union {
> +		struct {
> +			u32 cpu_id_start;
> +			u32 cpu_id;
> +		};
> +		u64 val;
> +	} x = { { .cpu_id_start = cpu_id, .cpu_id = cpu_id, } };
> +
> +	if (__put_user(x.val, (u64 *)&t->rseq->cpu_id_start))
> 		return -EFAULT;
> +
> 	trace_rseq_update(t);
> 	return 0;
>  }

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