[S] Change in coreboot[main]: arch/arm64/arch_timer: Fix possible overflow in multiplication

[Martin L Roth (Code Review)]

Martin L Roth has submitted this change. ( https://review.coreboot.org/c/coreboot/+/78800?usp=email )

Change subject: arch/arm64/arch_timer: Fix possible overflow in multiplication
......................................................................

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 
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78800
Reviewed-by: Yu-Ping Wu 
Tested-by: build bot (Jenkins) 
---
M src/arch/arm64/arch_timer.c
1 file changed, 20 insertions(+), 3 deletions(-)

Approvals:
  Yu-Ping Wu: Looks good to me, approved
  build bot (Jenkins): Verified

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);
 }