Commit 456e3788 authored by Adrian Hunter's avatar Adrian Hunter Committed by Thomas Gleixner
Browse files

vdso: Make delta calculation overflow safe 



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: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-8-adrian.hunter@intel.com
parent d2e58ab5

lib/vdso/gettimeofday.c

+16 −1
Original line number Diff line number Diff line
@@ -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);



	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 */