netdev - RE: bpf: undefined shift in __bpf_prog

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Date:	Mon, 7 Dec 2015 11:14:22 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Dmitry Vyukov' <dvyukov@...gle.com>,
	David Miller <davem@...emloft.net>
CC:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: RE: bpf: undefined shift in __bpf_prog_run

From: Dmitry Vyukov
> Sent: 04 December 2015 19:49
...
> 3.4.3
> undefined behavior
> 1 behavior, upon use of a nonportable or erroneous program construct
> or of erroneous data, for which this International Standard imposes no
> requirements
> 2 NOTE Possible undefined behavior ranges from ignoring the situation
> completely with unpredictable results, to behaving during translation
> or program execution in a documented manner characteristic of the
> environment (with or without the issuance of a diagnostic message), to
> terminating a translation or execution

While 'undefined behaviour' is allowed to include 'firing an ICBM at
the current location of the person who wrote the code' it is very
unlikely to result in anything other than an unexpected value
and the compiler making false assumptions about the value.

eg the compiler can assume this is an infinite loop:
	int i;
	for (i = 0; i >= 0; i++)
		...

	David