From: David Laight <David.Laight@ACULAB.COM>
To: 'Alan Stern' <stern@rowland.harvard.edu>,
Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>,
Jann Horn <jannh@google.com>,
Joel Fernandes <joel@joelfernandes.org>,
Kees Cook <keescook@chromium.org>,
"Eric W. Biederman" <ebiederm@xmission.com>,
LKML <linux-kernel@vger.kernel.org>,
Android Kernel Team <kernel-team@android.com>,
Kernel Hardening <kernel-hardening@lists.openwall.com>,
Andrew Morton <akpm@linux-foundation.org>,
Matthew Wilcox <willy@infradead.org>,
"Michal Hocko" <mhocko@suse.com>,
"Reshetova, Elena" <elena.reshetova@intel.com>
Subject: RE: [PATCH] Convert struct pid count to refcount_t
Date: Mon, 1 Apr 2019 15:28:35 +0000 [thread overview]
Message-ID: <76f3b894980d4a79a2c0ae815425d6a0@AcuMS.aculab.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1903291519060.1497-100000@iolanthe.rowland.org>
From: Alan Stern
> Sent: 29 March 2019 19:45
...
> There is a big difference between WRITE_ONCE() and plain assignment.
> Given "WRITE_ONCE(X, 2)", the compiler will emit a simple store
> instruction. But given "X = 2", the compiler is allowed to emit
> instructions equivalent to:
>
> if (X != 2)
> X = 2;
Worse for you, it can also emit:
X = 0;
X = 2;
Many years ago I fell foul of a compiler (not C) that implemented
a write to a 2 bit wide bitfield as:
X &= ~3
X |= value
even when 'value' was a compile time constant of 3.
Took a while to find out why the linked list got f*cked.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2019-04-01 15:27 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-27 14:53 Joel Fernandes (Google)
2019-03-28 0:06 ` Kees Cook
2019-03-28 0:59 ` Jann Horn
2019-03-28 2:34 ` Joel Fernandes
2019-03-28 2:57 ` Jann Horn
2019-03-28 14:37 ` Joel Fernandes
2019-03-28 15:17 ` Jann Horn
2019-03-28 16:26 ` Oleg Nesterov
2019-03-28 17:37 ` Paul E. McKenney
2019-03-29 17:32 ` Oleg Nesterov
2019-03-29 19:45 ` Alan Stern
2019-04-01 15:28 ` David Laight [this message]
2019-03-30 2:36 ` Joel Fernandes
2019-03-30 15:16 ` Alan Stern
2019-03-31 21:57 ` Paul E. McKenney
2019-03-31 21:55 ` Paul E. McKenney
2019-04-01 21:11 ` Joel Fernandes
2019-04-04 15:23 ` Paul E. McKenney
2019-04-04 16:01 ` Alan Stern
2019-04-04 18:08 ` Joel Fernandes
2019-04-04 18:19 ` Paul E. McKenney
2019-04-04 20:31 ` Joel Fernandes
2019-04-04 19:09 ` Alan Stern
2019-03-28 20:00 ` Joel Fernandes
2019-03-29 2:24 ` Joel Fernandes
2019-03-28 16:52 ` Kees Cook
2019-03-28 14:26 ` Oleg Nesterov
2019-03-28 14:39 ` Joel Fernandes
2019-03-29 2:34 ` Joel Fernandes
2019-03-29 17:37 ` Oleg Nesterov
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