On 12/3/24 7:46 PM, Peter Maydell wrote:
On Tue, 3 Dec 2024 at 16:25, Anastasia Belova <[email protected]> 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 <[email protected]> --- 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
