[3.13.y.z extended stable] Patch "kcmp: fix standard comparison bug" has been added to staging queue

Kamal Mostafa kamal at canonical.com
Wed Oct 8 22:14:10 UTC 2014

This is a note to let you know that I have just added a patch titled

    kcmp: fix standard comparison bug

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:


This patch is scheduled to be released in version

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see



>From 73b1b78d1909c3f0d7dd40e338a4658d92acdce1 Mon Sep 17 00:00:00 2001
From: Rasmus Villemoes <linux at rasmusvillemoes.dk>
Date: Tue, 9 Sep 2014 14:51:01 -0700
Subject: kcmp: fix standard comparison bug

commit acbbe6fbb240a927ee1f5994f04d31267d422215 upstream.

The C operator <= defines a perfectly fine total ordering on the set of
values representable in a long.  However, unlike its namesake in the
integers, it is not translation invariant, meaning that we do not have
"b <= c" iff "a+b <= a+c" for all a,b,c.

This means that it is always wrong to try to boil down the relationship
between two longs to a question about the sign of their difference,
because the resulting relation [a LEQ b iff a-b <= 0] is neither
anti-symmetric or transitive.  The former is due to -LONG_MIN==LONG_MIN
(take any two a,b with a-b = LONG_MIN; then a LEQ b and b LEQ a, but a !=
b).  The latter can either be seen observing that x LEQ x+1 for all x,
implying x LEQ x+1 LEQ x+2 ...  LEQ x-1 LEQ x; or more directly with the
simple example a=LONG_MIN, b=0, c=1, for which a-b < 0, b-c < 0, but a-c >

Note that it makes absolutely no difference that a transmogrying bijection
has been applied before the comparison is done.  In fact, had the
obfuscation not been done, one could probably not observe the bug
(assuming all values being compared always lie in one half of the address
space, the mathematical value of a-b is always representable in a long).
As it stands, one can easily obtain three file descriptors exhibiting the
non-transitivity of kcmp().

Side note 1: I can't see that ensuring the MSB of the multiplier is
set serves any purpose other than obfuscating the obfuscating code.

Side note 2:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>
#include <sys/syscall.h>

enum kcmp_type {
pid_t pid;

int kcmp(pid_t pid1, pid_t pid2, int type,
	 unsigned long idx1, unsigned long idx2)
	return syscall(SYS_kcmp, pid1, pid2, type, idx1, idx2);
int cmp_fd(int fd1, int fd2)
	int c = kcmp(pid, pid, KCMP_FILE, fd1, fd2);
	if (c < 0) {
	assert(0 <= c && c < 3);
	return c;
int cmp_fdp(const void *a, const void *b)
	static const int normalize[] = {0, -1, 1};
	return normalize[cmp_fd(*(int*)a, *(int*)b)];
#define MAX 100 /* This is plenty; I've seen it trigger for MAX==3 */
int main(int argc, char *argv[])
	int r, s, count = 0;
	int REL[3] = {0,0,0};
	int fd[MAX];
	pid = getpid();
	while (count < MAX) {
		r = open("/dev/null", O_RDONLY);
		if (r < 0)
		fd[count++] = r;
	printf("opened %d file descriptors\n", count);
	for (r = 0; r < count; ++r) {
		for (s = r+1; s < count; ++s) {
			REL[cmp_fd(fd[r], fd[s])]++;
	printf("== %d\t< %d\t> %d\n", REL[0], REL[1], REL[2]);
	qsort(fd, count, sizeof(fd[0]), cmp_fdp);
	memset(REL, 0, sizeof(REL));

	for (r = 0; r < count; ++r) {
		for (s = r+1; s < count; ++s) {
			REL[cmp_fd(fd[r], fd[s])]++;
	printf("== %d\t< %d\t> %d\n", REL[0], REL[1], REL[2]);
	return (REL[0] + REL[2] != 0);

Signed-off-by: Rasmus Villemoes <linux at rasmusvillemoes.dk>
Reviewed-by: Cyrill Gorcunov <gorcunov at openvz.org>
"Eric W. Biederman" <ebiederm at xmission.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
 kernel/kcmp.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/kcmp.c b/kernel/kcmp.c
index e30ac0f..0aa69ea 100644
--- a/kernel/kcmp.c
+++ b/kernel/kcmp.c
@@ -44,11 +44,12 @@ static long kptr_obfuscate(long v, int type)
 static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
-	long ret;
+	long t1, t2;

-	ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);
+	t1 = kptr_obfuscate((long)v1, type);
+	t2 = kptr_obfuscate((long)v2, type);

-	return (ret < 0) | ((ret > 0) << 1);
+	return (t1 < t2) | ((t1 > t2) << 1);

 /* The caller must have pinned the task */

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