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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2013-07-27 03:53:54 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2013-08-20 11:46:54 -0700
commit5a581b367b5df0531265311fc681c2abd377e5e6 (patch)
parent458fb381eacdd23366cfa2fbdf5a467848683e3a (diff)
jiffies: Avoid undefined behavior from signed overflow
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 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@linux.vnet.ibm.com> Cc: John Stultz <john.stultz@linaro.org> Cc: "David S. Miller" <davem@davemloft.net> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Eric Dumazet <eric.dumazet@gmail.com> Cc: Kevin Easton <kevin@guarana.org> [ paulmck: Included time_after64() and time_after_eq64(), as suggested by Eric Dumazet, also fixed commit message.] Reviewed-by: Josh Triplett <josh@joshtriplett.org>
1 files changed, 4 insertions, 4 deletions
diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 97ba4e78a37e..d235e88cfd7c 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) \