sched/x86: Fix overflow in cyc2ns_offset (9993bc63) · Commits · 戴 / test

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@google.com> Acked-by:

John Stultz <john.stultz@linaro.org> Acked-by:

Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Paul Turner <pjt@google.com> Cc: john stultz <johnstul@us.ibm.com> Link: http://lkml.kernel.org/r/20120310004027.19291.88460.stgit@dungbeetle.mtv.corp.google.com Signed-off-by:

Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/timer.h

+2 −6

Original line number	Diff line number	Diff line
		@@ -57,14 +57,10 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);

		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;
		}

arch/x86/kernel/tsc.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -620,7 +620,8 @@ 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);
		offset = ns_now - mult_frac(tsc_now, scale,
		(1UL << CYC2NS_SCALE_FACTOR));
		}

		sched_clock_idle_wakeup_event(0);

include/linux/kernel.h

+13 −0

Original line number	Diff line number	Diff line
		@@ -85,6 +85,19 @@
		} \
		)

		/*
		* 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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