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

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Message-ID: <20180703182837.GC2494@hirez.programming.kicks-ass.net>
Date:   Tue, 3 Jul 2018 20:28:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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 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.

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;
 }