linux-kernel - Re: [PATCH jiffies] Avoid undefined behavior from signed overflow

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Message-ID: <20130729153035.18805zox54aoyubf@chicago.guarana.org>
Date:	Mon, 29 Jul 2013 15:30:35 +1000
From:	caf@...rana.org
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, john.stultz@...aro.org,
	davem@...emloft.net, arnd@...db.de, mingo@...nel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH jiffies] Avoid undefined behavior from signed overflow

Quoting "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>:

> According to the C standard 3.4.3p3, overflow of a signed integer results
> in undefined behavior.  This commit therefore changes the definitions
> of time_after() and time_after_eq() to avoid this undefined behavior.
> The trick is that the subtraction is done using unsigned arithmetic,
> which according to 6.2.5p9 cannot overflow because it is defined as
> modulo arithmetic.  This has the added (though admittedly quite small)
> benefit of shortening two lines of code by four characters each.
>
> Note that the C standard considers the cast from signed to
> unsigned to be implementation-defined, see 6.3.1.3p3.  However, on a
> two-complement system, an implementation that defines anything other
> than a reinterpretation of the bits is free come to me, and I will be
> happy to act as a witness for its being committed to an insane asylum.
> (Although I have nothing against saturating arithmetic or signals in
> some cases, these things really should not be the default.)

Don't worry, the case from signed to unsigned is actually well-defined -
the relevant part is 6.3.1.3p2 (in C99):

> Otherwise, if the new type is unsigned, the value is converted by
> repeatedly adding or subtracting one more than the maximum value that
> can be represented in the new type until the value is in the range of
> the new type.

...which ends up just being reinterpretation of the bits on a two's
complement system, as you'd hope (after sign-extension to the width of
the target unsigned type, that is).  This actually means if you were
mad enough to implement C on a sign-magnitude system, you'd be forced to
do a non-trivial conversion in this case.

     - Kevin

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