[PATCH v2] target/arm: Add overflow check for gt_recalc_timer

[Leonid Komarianskyi]

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
-- 
2.25.1