[PATCH v3] smp: Fix a potential usage of stale nr_cpus

[Muchun Song]

When the cmdline of "nr_cpus" is not valid, the @nr_cpu_ids is assigned
a stale value. The nr_cpus is only valid when get_option() return 1. So
check the return value to prevent this.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
changelog in v3:
 1) Return -EINVAL when the parameter is bogus. 

changelog in v2:
 1) Rework the commit log.
 2) Rework the return value check.

 kernel/smp.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index a5a66fc28f4e..0dacfcfcf00b 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -772,9 +772,13 @@ static int __init nrcpus(char *str)
 {
 	int nr_cpus;
 
-	get_option(&str, &nr_cpus);
+	if (get_option(&str, &nr_cpus) != 1)
+		return -EINVAL;
+
 	if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
 		nr_cpu_ids = nr_cpus;
+	else
+		return -EINVAL;
 
 	return 0;
 }
-- 
2.11.0