[RFC PATCH] sched: Fix integer overflow issue of cpu weight delta

[I Hsin Cheng <richard120310@gmail.com>]

In the original calculation of "cpu_weight_nice_read_s64()", the type of
"delta" is set as "int", while "weight" is of type "unsigned long". the
actual difference between "sched_prio_to_weight[prio]" and "weight"
could go beyond the limit of integer when "weight" is large enough.

For example if "weight" is "2147555403" or larger, the correct "prio"
should be "1", however, in the origin implementation , the result will
be "2" due to integer overflow problem, because "71755 - 2147555403 =
-2147483648", which is exactly the minimum value of integer, the macro
"abs()" won't be able to generate an "int" value with "2147483648".

Obviously, for all the weight larger than "sched_prio_to_weight[0]", the
"prio" result should be "1".

Change the type of "last_delta" and "delta" to "long" can solve the
issue.

I think there's one point to concern, do we have a fixed range of
"tg_weight()" ? if it can go beyond "2147555403", than I think this is
worth to be fixed.

Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
---
 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 95e40895a519..be52a2fff1e3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9917,8 +9917,8 @@ static s64 cpu_weight_nice_read_s64(struct cgroup_subsys_state *css,
 				    struct cftype *cft)
 {
 	unsigned long weight = tg_weight(css_tg(css));
-	int last_delta = INT_MAX;
-	int prio, delta;
+	long last_delta = LONG_MAX, delta;
+	int prio;
 
 	/* find the closest nice value to the current weight */
 	for (prio = 0; prio < ARRAY_SIZE(sched_prio_to_weight); prio++) {
-- 
2.43.0