[ 35/42] sched/x86: Fix overflow in cyc2ns_offset

From: Greg KH
Date: Wed Apr 11 2012 - 19:32:13 EST


3.0-stable review patch. If anyone has any objections, please let me know.

------------------

From: Salman Qazi <sqazi@xxxxxxxxxx>

commit 9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 upstream.

When a machine boots up, the TSC generally gets reset. However,
when kexec is used to boot into a kernel, the TSC value would be
carried over from the previous kernel. The computation of
cycns_offset in set_cyc2ns_scale is prone to an overflow, if the
machine has been up more than 208 days prior to the kexec. The
overflow happens when we multiply *scale, even though there is
enough room to store the final answer.

We fix this issue by decomposing tsc_now into the quotient and
remainder of division by CYC2NS_SCALE_FACTOR and then performing
the multiplication separately on the two components.

Refactor code to share the calculation with the previous
fix in __cycles_2_ns().

Signed-off-by: Salman Qazi <sqazi@xxxxxxxxxx>
Acked-by: John Stultz <john.stultz@xxxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: john stultz <johnstul@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20120310004027.19291.88460.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
arch/x86/include/asm/timer.h | 8 ++------
arch/x86/kernel/tsc.c | 3 ++-
include/linux/kernel.h | 13 +++++++++++++
3 files changed, 17 insertions(+), 7 deletions(-)

--- a/arch/x86/include/asm/timer.h
+++ b/arch/x86/include/asm/timer.h
@@ -57,14 +57,10 @@ DECLARE_PER_CPU(unsigned long long, cyc2

static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
{
- unsigned long long quot;
- unsigned long long rem;
int cpu = smp_processor_id();
unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
- quot = (cyc >> CYC2NS_SCALE_FACTOR);
- rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
- ns += quot * per_cpu(cyc2ns, cpu) +
- ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
+ ns += mult_frac(cyc, per_cpu(cyc2ns, cpu),
+ (1UL << CYC2NS_SCALE_FACTOR));
return ns;
}

--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -623,7 +623,8 @@ static void set_cyc2ns_scale(unsigned lo

if (cpu_khz) {
*scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
- *offset = ns_now - (tsc_now * *scale >> CYC2NS_SCALE_FACTOR);
+ *offset = ns_now - mult_frac(tsc_now, *scale,
+ (1UL << CYC2NS_SCALE_FACTOR));
}

sched_clock_idle_wakeup_event(0);
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -77,6 +77,19 @@
} \
)

+/*
+ * Multiplies an integer by a fraction, while avoiding unnecessary
+ * overflow or loss of precision.
+ */
+#define mult_frac(x, numer, denom)( \
+{ \
+ typeof(x) quot = (x) / (denom); \
+ typeof(x) rem = (x) % (denom); \
+ (quot * (numer)) + ((rem * (numer)) / (denom)); \
+} \
+)
+
+
#define _RET_IP_ (unsigned long)__builtin_return_address(0)
#define _THIS_IP_ ({ __label__ __here; __here: (unsigned long)&&__here; })



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