[PATCH 07/19] vdso: Make delta calculation overflow safe

[Adrian Hunter]

Kernel timekeeping is designed to keep the change in cycles (since the last
timer interrupt) below max_cycles, which prevents multiplication overflow
when converting cycles to nanoseconds. However, if timer interrupts stop,
the calculation will eventually overflow.

Add protection against that, enabled by config option
CONFIG_GENERIC_VDSO_OVERFLOW_PROTECT. Check against max_cycles, falling
back to a slower higher precision calculation.

Suggested-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter at intel.com>
---
 lib/vdso/gettimeofday.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 9fa90e0794c9..9c3a8d2440c9 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -13,6 +13,18 @@
 # define VDSO_DELTA_MASK(vd)	(vd->mask)
 #endif
 
+#ifdef CONFIG_GENERIC_VDSO_OVERFLOW_PROTECT
+static __always_inline bool vdso_delta_ok(const struct vdso_data *vd, u64 delta)
+{
+	return delta < vd->max_cycles;
+}
+#else
+static __always_inline bool vdso_delta_ok(const struct vdso_data *vd, u64 delta)
+{
+	return true;
+}
+#endif
+
 #ifndef vdso_shift_ns
 static __always_inline u64 vdso_shift_ns(u64 ns, u32 shift)
 {
@@ -28,7 +40,10 @@ static __always_inline u64 vdso_calc_ns(const struct vdso_data *vd, u64 cycles,
 {
 	u64 delta = (cycles - vd->cycle_last) & VDSO_DELTA_MASK(vd);
 
-	return vdso_shift_ns((delta * vd->mult) + base, vd->shift);
+	if (likely(vdso_delta_ok(vd, delta)))
+		return vdso_shift_ns((delta * vd->mult) + base, vd->shift);
+
+	return mul_u64_u32_add_u64_shr(delta, vd->mult, base, vd->shift);
 }
 #endif /* vdso_calc_ns */
 
-- 
2.34.1