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Subject[PATCH] sched, x86: fix overflow in cyc2ns_offset
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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.

Signed-off-by: Salman Qazi <sqazi@google.com>
---
arch/x86/kernel/tsc.c | 12 +++++++++++-
1 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index a62c201..ef1dc8e 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -608,6 +608,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
{
unsigned long long tsc_now, ns_now, *offset;
unsigned long flags, *scale;
+ unsigned long long quot;
+ unsigned long long rem;

local_irq_save(flags);
sched_clock_idle_sleep_event();
@@ -620,7 +622,15 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)

if (cpu_khz) {
*scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
- *offset = ns_now - (tsc_now * *scale >> CYC2NS_SCALE_FACTOR);
+
+ /*
+ * Avoid premature overflow by splitting into quotient
+ * and remainder. See the comment above __cycles_2_ns
+ */
+ quot = (tsc_now >> CYC2NS_SCALE_FACTOR);
+ rem = tsc_now & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
+ *offset = ns_now - (quot * *scale +
+ ((rem * *scale) >> CYC2NS_SCALE_FACTOR));
}

sched_clock_idle_wakeup_event(0);


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