linux-kernel - Re: [PATCHv2 02/10] kernel: Provide READ_ONCE and ASSIGN

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20141125175036.GH5050@linux.vnet.ibm.com>
Date:	Tue, 25 Nov 2014 09:50:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	David Howells <dhowells@...hat.com>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCHv2 02/10] kernel: Provide READ_ONCE and ASSIGN_ONCE

On Tue, Nov 25, 2014 at 09:28:33AM -0800, Linus Torvalds wrote:
> On Tue, Nov 25, 2014 at 7:59 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > We really need something like this to catch invalid sizes:
> >
> >         default: invoke_nonexistent_function();
> 
> Actually, I wonder if we should make the default: case actually just
> do something like
> 
>    barrier();
>    memcpy(res, p, size);
>    barrier();
> 
> which in no way guarantees that it's an _atomic_ access, but it does
> guarantee the semantics that you get one particular value and it won't
> get reloaded later..
> 
> That would solve the crazy sparc pte issue too.

I would be really worried about confusion due to load/store tearing,
where a READ_ONCE() reads part of its value from one ASSIGN_ONCE()
and the other part from some other ASSIGN_ONCE().  Don't get me wrong,
there are cases where the load/store tearing is harmless, it is just
that in my experience that these cases are anything but the common case.

That said, I do not claim to be familiar with more than a microscopic
fraction of the Linux kernel.

Of course, one way to resolve this would be to have one variant that did
the memcpy() and another that threw a build error, maybe READ_ONCE_FORCE()
and ASSIGN_ONCE_FORC() or some such.  I would -really- like to be informed
if I do READ_ONCE() of a long long on a 32-bit system.  ;-)

/me goes off to see if there are any ACCESS_ONCE() of long longs in RCU...

							Thanx, Paul

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/