[PATCH V2 07/19] vdso: Make delta calculation overflow safe
From: Adrian Hunter <adrian.hunter@intel.com>
Date: 2024-03-25 06:41:26
Also in:
linux-s390, lkml
Subsystem:
generic vdso library, library code, the rest · Maintainers:
Andy Lutomirski, Thomas Gleixner, Vincenzo Frascino, Andrew Morton, Linus Torvalds
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 <redacted> Signed-off-by: Adrian Hunter <adrian.hunter@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