linux-kernel - Re: [PATCH for 4.18 2/6] rseq: use get_user/put_user rather than __get_user/__put

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Date:   Mon, 9 Jul 2018 14:19:19 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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>,
        Andy Lutomirski <luto@...capital.net>,
        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>,
        Andrew Hunter <ahh@...gle.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH for 4.18 2/6] rseq: use get_user/put_user rather than
 __get_user/__put_user

----- On Jul 9, 2018, at 2:04 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Mon, Jul 9, 2018 at 10:28 AM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> So, another twist to this story: ppc32 does not implement u64 get_user().
> 
> I was going to say that "that's not possible", since we actually have
> 64-bit arguments at least in the form of "loff_t __user *".
> 
> But when I started looking, it turns out that yeah, we do
> "copy_from_user()" on them, and instead made the x86 copy_from_user()
> have special cases for constant sizes.
> 
> So a 8-byte copy_from_user() is fine. It ends up being a "get_user()"
> on x86 anyway.

Given that this memory area has already been checked with access_ok()
on rseq registration, are you fine with leaving
__get_user/__put_user/__copy_{from,to}_user in place so we do the
minimal change at this stage of rc, or should I go ahead and replace
the lot with get_user/put_user/copy_{from,to}_user ?

Thanks,

Mathieu


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