Re: [Qemu-devel] [PATCH v2] timer: a9gtimer: check auto-increment register value

[Peter Maydell <peter.maydell@linaro.org>]

On 28 September 2016 at 02:16, P J P <ppandit@redhat.com> wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
>
> ARM A9MP processor has a peripheral timer with an auto-increment
> register, which holds an increment step value. A user could set
> this value to zero, when auto-increment control bit is enabled.
> This leads to an infinite loop in 'a9_gtimer_update' while
> updating comparator value. Remove the loop incrementing the
> comparator value; Add check to avoid value zero.
>
> Reported-by: Li Qiang <liqiang6-s@360.cn>
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  hw/timer/a9gtimer.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
>
> Update per
>   -> https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg05682.html
>
> diff --git a/hw/timer/a9gtimer.c b/hw/timer/a9gtimer.c
> index 772f85f..021ae59 100644
> --- a/hw/timer/a9gtimer.c
> +++ b/hw/timer/a9gtimer.c
> @@ -72,7 +72,7 @@ static void a9_gtimer_update(A9GTimerState *s, bool sync)
>  {
>
>      A9GTimerUpdate update = a9_gtimer_get_update(s);
> -    int i;
> +    uint32_t i, inc;

Why have you changed the type of i when you haven't changed
any of the code that works with it?

Since the gtb->compare field is 64 bit your inc variable should
probably be uint64_t.

>      int64_t next_cdiff = 0;
>
>      for (i = 0; i < s->num_cpu; ++i) {
> @@ -82,16 +82,14 @@ static void a9_gtimer_update(A9GTimerState *s, bool sync)
>          if ((s->control & R_CONTROL_TIMER_ENABLE) &&
>                  (gtb->control & R_CONTROL_COMP_ENABLE)) {
>              /* R2p0+, where the compare function is >= */
> -            while (gtb->compare < update.new) {
> -                DB_PRINT("Compare event happened for CPU %d\n", i);
> -                gtb->status = 1;
> -                if (gtb->control & R_CONTROL_AUTO_INCREMENT) {
> -                    DB_PRINT("Auto incrementing timer compare by %" PRId32 "\n",
> -                             gtb->inc);
> -                    gtb->compare += gtb->inc;
> -                } else {
> -                    break;
> -                }
> +            DB_PRINT("Compare event happened for CPU %d\n", i);

This is wrong -- this debug print should only happen when the
comparator matches (ie when gtb->compare is less than update.new),
not every time.

> +            gtb->status = 1;

Similarly the status flag should only be set when the comparator
matches.

> +            if (gtb->control & R_CONTROL_AUTO_INCREMENT
> +                && gtb->compare < update.new) {
> +                inc = ((update.new - gtb->compare - 1) / gtb->inc) * gtb->inc;

I suggest using either QEMU_ALIGN_UP or QEMU_ALIGN_DOWN here,
depending on which way round you're trying for.

Also this will divide by zero if gtb->inc is 0 (as for instance
it will be on reset).

> +                DB_PRINT("Auto incrementing timer compare by %"
> +                                                            PRId32 "\n", inc);
> +                gtb->compare += inc;
>              }
>              cdiff = (int64_t)gtb->compare - (int64_t)update.new + 1;
>              if (cdiff > 0 && (cdiff < next_cdiff || !next_cdiff)) {
> @@ -210,7 +208,7 @@ static void a9_gtimer_write(void *opaque, hwaddr addr, uint64_t value,
>          gtb->compare = deposit64(gtb->compare, shift, 32, value);
>          break;
>      case R_AUTO_INCREMENT:
> -        gtb->inc = value;
> +        gtb->inc = value ? value : 1;

Where has this change come from ??

>          return;
>      default:
>          return;
> --
> 2.5.5
>

thanks
-- PMM