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:   Tue, 10 Jul 2018 16:16:28 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     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>,
        Linus Torvalds <torvalds@...ux-foundation.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

Mathieu Desnoyers <mathieu.desnoyers@...icios.com> writes:
> ----- On Jul 8, 2018, at 5:03 PM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:
>
>> In preparation to use __u64 for the rseq_cs pointer field, 32-bit
>> architectures need to read this 64-bit value located in user-space
>> addresses.
>> 
>> __get_user is used to read this value, given that its access check has
>> already been performed with access_ok() on rseq registration.
>> 
>> arm does not implement 8-byte __get_user. Rather than trying to
>> improve __get_user on ARM, use get_user/put_user across rseq instead.
>> 
>> If those end up showing up in benchmarks, the proper approach would be to
>> use user_access_begin() / unsafe_get/put_user() / user_access_end()
>> anyway.
>
> So, another twist to this story: ppc32 does not implement u64 get_user().

Or __get_user() for that matter.

But we should just fix it.

We have the asm to do it, it's just the fact that __gu_val is unsigned
long causes the size > sizeof(x) check here to fail:

#define __get_user_size(x, ptr, size, retval)			\
do {								\
	retval = 0;						\
	__chk_user_ptr(ptr);					\
	if (size > sizeof(x))					\
		(x) = __get_user_bad();				\



We seem to be able to fix that with the __inttype() trick that x86 uses.

That's probably not 4.18 material though. But if you want to go with
copy_from_user() for now you could then switch to get_user() for 4.19.

cheers