Date: Thu, 23 Jul 2015 11:02:55 -0700
From: =?iso-8859-1?Q?J=F6rn?= Engel <joern@purestorage.com>
To: linux-kernel@vger.kernel.org, Yinghai Lu <yinghai@kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>
Cc: Spencer Baugh <spencer.baugh@purestorage.com>
Subject: round_up integer underflow
Message-ID: <20150723180255.GA24876@Sligo.logfs.org>
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Spencer spotted something nasty in the round_up macro.  We were
wondering why round_up() worked differently from ALIGN.  The only real
difference between the two patterns is overflow behaviour.  And both
version are buggy when used for signed integer types, round_up will
underflow on INT_MIN, ALIGN will overflow on INT_MAX.  Since signed
integer under/overflows are undefined, we might have subtle bugs lurking
in the kernel.

This example program produces a warning when compiling with gcc -O2 or
higher.  Clang doesn't warn.  Compiled code behaves correctly with both
compilers, but that is largely luck and the same compilers may create
wrong behaviour if the surrounding code changes.

#include <limits.h>
#include <stdio.h>

#define __round_mask(x, y) ((__typeof__(x))((y)-1))
#define round_up(x, y) ((((x)-1) | __round_mask(x, y))+1)
#define round_down(x, y) ((x) & ~__round_mask(x, y))

int main(void)
{
	int i, r = 8;

	for (i = INT_MIN; i; i++) {
		printf("%2x: %2x %2x\n", i, round_down(i, r), round_up(i, r));
	}
	return 0;
}

I don't have a good answer yet.  We could make round_up check for
negative numbers, but I would prefer unconditional code that optimizes
down to nothing.  We could rewrite it in assembly, once for each
architecture.

Does anyone have better ideas?

J?rn

--
People really ought to be forced to read their code aloud over the phone.
That would rapidly improve the choice of identifiers.
-- Al Viro
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