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From: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: Re: Smatch check for Spectre stuff
Date: Wed, 25 Apr 2018 15:48:52 +0100	[thread overview]
Message-ID: <20180425154852.2486f267@alans-desktop> (raw)
In-Reply-To: <20180425131958.hhapvc3b2i3b4pgy@lakrids.cambridge.arm.com>

> 2) Compiler transformations can elide binary operations, so we cannot
>    rely on source level AND (&) or MOD (%) operations to narrow the
>    range of an expression, regardless of the types of either operand.
> 
>    This means that source-level AND and MOD operations cannot be relied
>    upon under speculation.

You need to use volatiles and memory barriers if trying to do it
explicitly in C. The compilers will do some really quite insanely
brilliant things otherwise. That's one reason that not using fences is
really tricky and belongs wrapped in helpers.

> I suspect this means *many* more potential spectre gadgets. :(

I expect so as well as probably a lot of false positives - the tools in
the space are all pretty new.

Array access isn't always needed either. Remember that something as
simple as

	x = size_table[user];
	memset(buf, 0, x);

can speculatively reveal things, as can 'classical' side channels such as
variable length instruction timings.

Alan

  reply	other threads:[~2018-04-25 14:49 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-19  5:15 Dan Carpenter
2018-04-19 21:39 ` Gustavo A. R. Silva
2018-04-20 12:00 ` Peter Zijlstra
2018-04-23 12:31   ` Gustavo A. R. Silva
2018-04-23 12:45     ` Peter Zijlstra
2018-04-23 13:08       ` Gustavo A. R. Silva
2018-04-23 13:48       ` Dan Williams
2018-04-20 12:25 ` Thomas Gleixner
2018-04-20 17:21   ` Oleg Nesterov
2018-04-20 12:47 ` Mark Rutland
2018-04-23 12:53   ` Dan Carpenter
2018-04-23 13:22     ` Mark Rutland
2018-04-23 13:26       ` Dan Carpenter
2018-04-23 17:11 ` Davidlohr Bueso
2018-04-25 13:19 ` Mark Rutland
2018-04-25 14:48   ` Alan Cox [this message]
2018-04-25 15:03     ` Mark Rutland
2018-06-08 16:12 ` Josh Poimboeuf
2018-06-11  9:28   ` Peter Zijlstra
2018-06-13 13:10   ` Dan Carpenter
2018-06-13 13:58     ` Dan Carpenter

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