Re: [PATCH] hw/timer/nrf51_timer: prevent integer overflow

[Anastasia Belova]

On 12/3/24 7:46 PM, Peter Maydell wrote:
> On Tue, 3 Dec 2024 at 16:25, Anastasia Belova <abelova@astralinux.ru> wrote:
> > Both counter and tick are uint32_t and the result
> > of their addition may not fit this type. Add
> > explicit casting to uint64_t.
> >
> > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> >
> > Fixes: c5a4829c08 ("hw/timer/nrf51_timer: Add nRF51 Timer peripheral")
> > Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
> > ---
> >   hw/timer/nrf51_timer.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/timer/nrf51_timer.c b/hw/timer/nrf51_timer.c
> > index 35b0e62d5b..b5ff235eb8 100644
> > --- a/hw/timer/nrf51_timer.c
> > +++ b/hw/timer/nrf51_timer.c
> > @@ -44,7 +44,7 @@ static uint32_t update_counter(NRF51TimerState *s, int64_t 
> > now)
> >   {
> >       uint32_t ticks = ns_to_ticks(s, now - s->update_counter_ns);
> >
> > -    s->counter = (s->counter + ticks) % BIT(bitwidths[s->bitmode]);
> > +    s->counter = ((uint64_t)s->counter + ticks) % BIT(bitwidths[s->bitmode]);
> Can you explain when adding the cast makes a difference?
> Since s->counter is 32 bits and ticks is 32 bits and
> the RHS of the % is a power of 2, it's not clear to
> me that keeping the top 32 bits in the addition and then
> discarding it after the % is any different from only
> taking the bottom 32 bits of the addition.
You're right. I was sure this situation invokes UB.

Thanks for your patience,
Anastasia Belova