arch/arm64/arch_timer: Fix possible overflow in multiplication
The value from raw_read_cntfrq_el0() could be large enough to cause
overflow when multiplied by USECS_PER_SEC. To prevent this, both
USECS_PER_SEC and tfreq can be reduced by dividing them by their GCD.
BUG=b:307790895
TEST=emerge-geralt coreboot
TEST=boot to kernel and check the timestamps from `cbmem`
Change-Id: I366667de05392913150414f0fa9058725be71c52
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78800
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/arch/arm64/arch_timer.c b/src/arch/arm64/arch_timer.c
index 089afee..3eb5656 100644
--- a/src/arch/arm64/arch_timer.c
+++ b/src/arch/arm64/arch_timer.c
@@ -1,12 +1,29 @@
/* SPDX-License-Identifier: GPL-2.0-only */
-#include <timer.h>
#include <arch/lib_helpers.h>
+#include <commonlib/bsd/gcd.h>
+#include <timer.h>
void timer_monotonic_get(struct mono_time *mt)
{
uint64_t tvalue = raw_read_cntpct_el0();
- uint32_t tfreq = raw_read_cntfrq_el0();
- long usecs = (tvalue * 1000000) / tfreq;
+ static uint32_t tfreq, mult;
+ uint32_t div;
+
+ /*
+ * The value from raw_read_cntfrq_el0() could be large enough to
+ * cause overflow when multiplied by USECS_PER_SEC. To prevent this,
+ * both USECS_PER_SEC. and tfreq can be reduced by dividing them by
+ * their GCD.
+ */
+ if (tfreq == 0) {
+ tfreq = raw_read_cntfrq_el0();
+ mult = USECS_PER_SEC;
+ div = gcd32(tfreq, mult);
+ tfreq /= div;
+ mult /= div;
+ }
+
+ long usecs = (tvalue * mult) / tfreq;
mono_time_set_usecs(mt, usecs);
}