[3.11.y.z extended stable] Patch "jiffies: Avoid undefined behavior from signed overflow" has been added to staging queue

[Luis Henriques]

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

    jiffies: Avoid undefined behavior from signed overflow

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

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

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From b7cce180bc60308bf14f42f459bb4b96c7b7145b Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
Date: Sat, 27 Jul 2013 03:53:54 -0700
Subject: jiffies: Avoid undefined behavior from signed overflow

commit 5a581b367b5df0531265311fc681c2abd377e5e6 upstream.

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(), time_after_eq(), time_after64(), and time_after_eq64()
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 four lines of code
by four characters each.

Note that the C standard considers the cast from unsigned to
signed to be implementation-defined, see 6.3.1.3p3.  However, on a
two's-complement system, an implementation that defines anything other
than a reinterpretation of the bits is free to 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 when compiling an
operating-system kernel.)

Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
Cc: John Stultz <john.stultz at linaro.org>
Cc: "David S. Miller" <davem at davemloft.net>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Ingo Molnar <mingo at kernel.org>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Eric Dumazet <eric.dumazet at gmail.com>
Cc: Kevin Easton <kevin at guarana.org>
[ paulmck: Included time_after64() and time_after_eq64(), as suggested
  by Eric Dumazet, also fixed commit message.]
Reviewed-by: Josh Triplett <josh at joshtriplett.org>
Cc: Ruchi Kandoi <kandoiruchi at google.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 include/linux/jiffies.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 97ba4e7..d235e88 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -101,13 +101,13 @@ static inline u64 get_jiffies_64(void)
 #define time_after(a,b)		\
 	(typecheck(unsigned long, a) && \
 	 typecheck(unsigned long, b) && \
-	 ((long)(b) - (long)(a) < 0))
+	 ((long)((b) - (a)) < 0))
 #define time_before(a,b)	time_after(b,a)

 #define time_after_eq(a,b)	\
 	(typecheck(unsigned long, a) && \
 	 typecheck(unsigned long, b) && \
-	 ((long)(a) - (long)(b) >= 0))
+	 ((long)((a) - (b)) >= 0))
 #define time_before_eq(a,b)	time_after_eq(b,a)

 /*
@@ -130,13 +130,13 @@ static inline u64 get_jiffies_64(void)
 #define time_after64(a,b)	\
 	(typecheck(__u64, a) &&	\
 	 typecheck(__u64, b) && \
-	 ((__s64)(b) - (__s64)(a) < 0))
+	 ((__s64)((b) - (a)) < 0))
 #define time_before64(a,b)	time_after64(b,a)

 #define time_after_eq64(a,b)	\
 	(typecheck(__u64, a) && \
 	 typecheck(__u64, b) && \
-	 ((__s64)(a) - (__s64)(b) >= 0))
+	 ((__s64)((a) - (b)) >= 0))
 #define time_before_eq64(a,b)	time_after_eq64(b,a)

 #define time_in_range64(a, b, c) \
--
1.9.0