linux-kernel - Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

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Date:   Wed, 28 Mar 2018 10:31:39 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable
 sequences system call (v12)

----- On Mar 28, 2018, at 10:06 AM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:

> ----- On Mar 28, 2018, at 2:47 AM, Boqun Feng boqun.feng@...il.com wrote:
> 
>> On Tue, Mar 27, 2018 at 12:05:23PM -0400, Mathieu Desnoyers wrote:
>> [...]
>>> Changes since v11:
>>> 
>>> - Replace task struct rseq_preempt, rseq_signal, and rseq_migrate
>>>   bool by u32 rseq_event_mask.
>> [...]
>>> @@ -979,6 +980,17 @@ struct task_struct {
>>>  	unsigned long			numa_pages_migrated;
>>>  #endif /* CONFIG_NUMA_BALANCING */
>>>  
>>> +#ifdef CONFIG_RSEQ
>>> +	struct rseq __user *rseq;
>>> +	u32 rseq_len;
>>> +	u32 rseq_sig;
>>> +	/*
>>> +	 * RmW on rseq_event_mask must be performed atomically
>>> +	 * with respect to preemption.
>>> +	 */
>>> +	unsigned long rseq_event_mask;
>> 
>> s/unsigned long/u32
> 
> good point, fixed.
> 

Actually, by having a u32 instead of unsigned long here, it triggers those
warnings:

In file included from ./include/linux/bitops.h:38:0,
                 from ./include/linux/kernel.h:11,
                 from certs/system_keyring.c:13:
./arch/x86/include/asm/bitops.h:73:1: note: expected ‘volatile long unsigned int *’ but argument is of type ‘u32 *’
 set_bit(long nr, volatile unsigned long *addr)
 ^

I suspect that casting the u32 * to a unsigned long * is not a safe approach, because
the code can generate a load/store on unallocated memory (kasan might complain).

Thoughts ?

Thanks,

Mathieu


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