[PATCH v3] rcu: Fix buffer overlow in print_cpu_stall_info()

[Nikita Kiryushin]

rcuc info output in print_cpu_stall_info() contains
posiible buffer overflow in the case of huge jiffies
difference. The situation seems improbable, but, buffer
overflow, still.

Also, unsigned jiffies difference printed as (signed)
%ld. This is intentional for debugging purposes, but
it is not obvious from the code.

Change sprintf to snprintf and add clarifying comment
about intention of %ld format.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 245a62982502 ("rcu: Dump rcuc kthread status for CPUs not reporting quiescent state")
Signed-off-by: Nikita Kiryushin <kiryushin@xxxxxxxx>
---
v3: Change intention comment wording as
Steven Rostedt <rostedt@xxxxxxxxxxx> suggested
v2: Remove signed to unsigned print format change as
Paul E. McKenney <paulmck@xxxxxxxxxx> suggested, add format
intention clarification comment
 kernel/rcu/tree_stall.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 5d666428546b..320440b8384e 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -504,7 +504,8 @@ static void print_cpu_stall_info(int cpu)
 			rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
 	rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
 	if (rcuc_starved)
-		sprintf(buf, " rcuc=%ld jiffies(starved)", j);
+		/* Print signed value, as negative means it is likely a bug */
+		snprintf(buf, sizeof(buf), " rcuc=%ld jiffies(starved)", j);
 	pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
 	       cpu,
 	       "O."[!!cpu_online(cpu)],
-- 
2.34.1