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Re: [PATCH v2] target/arm: Add overflow check for gt_recalc_timer
From: |
Leonid Komarianskyi |
Subject: |
Re: [PATCH v2] target/arm: Add overflow check for gt_recalc_timer |
Date: |
Thu, 9 Nov 2023 15:13:58 +0000 |
Peter Maydell, thank you for your comments.
I apologize for so late response - returned to this issue and now I will
answer faster. I fixed the commit according to your recommendations,
please take a look at the new version.
> There is also a comment or two from me in the bug report pointing
> out that the handling of wraparound is also wrong in the other
> half of this if(); we should look at that too.
I read this topic and as I understand changing the other half of "if" is
not related to the reported issue. Since it affects running virtualized
setups on arm64 QEMU, e.g. Zephyr
(https://github.com/zephyrproject-rtos/zephyr/blob/main//boards/arm64/xenvm/doc/index.rst)
maybe is it worth merging at least this change?
Best regards,
Leonid Komarianskyi.
On 09.11.23 15:55, Leonid Komarianskyi wrote:
> If gt_timer is enabled before cval initialization on a virtualized
> setup on QEMU, cval equals (UINT64_MAX - 1). Adding an offset value
> to this causes an overflow that sets timer into the past, which leads
> to infinite loop, because this timer fires immediately and calls
> gt_recalc_timer() once more, which in turn sets the timer into the
> past again and as a result, QEMU hangs. This patch adds check for
> overflowing of the nexttick variable.
>
> Suggested-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> Co-Authored-By: Dmytro Firsov <dmytro_firsov@epam.com>
> Signed-off-by: Leonid Komarianskyi <leonid_komarianskyi@epam.com>
> ---
> target/arm/helper.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index 3b22596eab..b4aaa2965b 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -2665,6 +2665,16 @@ static void gt_recalc_timer(ARMCPU *cpu, int timeridx)
> } else {
> /* Next transition is when we hit cval */
> nexttick = gt->cval + offset;
> + if (nexttick < gt->cval) {
> + /*
> + * If gt->cval value is close to UINT64_MAX then adding
> + * to it offset can lead to overflow of nexttick variable.
> + * So, this check tests that arguments sum is less than any
> + * addend, and in case it is overflowed we have to mod timer
> + * to INT64_MAX.
> + */
> + nexttick = UINT64_MAX;
> + }
> }
> /*
> * Note that the desired next expiry time might be beyond the