[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <1048940999.11846.1530640717837.JavaMail.zimbra@efficios.com>
Date: Tue, 3 Jul 2018 13:58:37 -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 1:48 PM, Peter Zijlstra peterz@...radead.org wrote:
> On Tue, Jul 03, 2018 at 01:38:59PM -0400, Mathieu Desnoyers wrote:
>> ----- On Jul 3, 2018, at 1:34 PM, Peter Zijlstra peterz@...radead.org wrote:
>>
>> > On Tue, Jul 03, 2018 at 10:10:37AM -0700, Linus Torvalds wrote:
>> >> On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen <andi@...stfloor.org> wrote:
>> >> >
>> >> > So it sounds like architectures that don't have an instruction atomic u64
>> >> > *_user need to disable interrupts during the access, and somehow handle that
>> >> > case when a page fault happens?
>> >>
>> >> No. It's actually the store by *user* space that is the critical one.
>> >> Not the whole 64-bit value, just the low pointer part.
>> >>
>> >> The kernel could do it as a byte-by-byte load, really. It's
>> >> per-thread, and once the kernel is running, it's not going to change.
>> >> The kernel never changes the value, it just loads it from user space.
>> >
>> > The kernel doesn't change _this_ value, but the kernel does change other
>> > values, like for instance rseq->cpu_id. But even there, it could use
>> > byte stores and it is again the userspace load of that field that is
>> > critical again and needs to be a single op.
>>
>> I can simply document that loads/stores from/to all struct rseq fields
>> should be thread-local then ?
>
> I'm not sure that covers things sufficiently. You really want the
> userspace load/stores to be single instructions.
Yes, of course. More specifically, I would document that those need to
be single-copy atomicity load/store performed by the local thread.
> Also, I think it was rseq_update_cpu_id() where we wanted to use a
> single u64 store if possible but you worried about the stores.
With this added bit of restriction on thread-local loads, indeed we
can then update them without caring about atomicity at kernel level.
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 ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
Powered by blists - more mailing lists