[PATCH] kernel/sys.c: use RCU when accessing task_struct->real_parent

[Alex Elder <elder@riscstar.com>]

In the setpgid() syscall definition, a check is made to determine
whether the target process is in the same thread group as the
current process.  The check accesses the target process's real_parent
pointer directly, however that field is supposed to be accessed via
RCU.  Use rcu_dereference() to avoid this C=1 build warning:

  kernel/sys.c:1144:32: warning: incorrect type in argument 1 (different address spaces)

Signed-off-by: Alex Elder <elder@riscstar.com>
---
 kernel/sys.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index df69bd71de032..434fe6ad30e13 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1114,6 +1114,7 @@ COMPAT_SYSCALL_DEFINE1(times, struct compat_tms __user *, tbuf)
 SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 {
 	struct task_struct *p;
+	struct task_struct *real_parent;
 	struct task_struct *group_leader = current->group_leader;
 	struct pid *pids[PIDTYPE_MAX] = { 0 };
 	struct pid *pgrp;
@@ -1141,7 +1142,8 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 	if (!thread_group_leader(p))
 		goto out;
 
-	if (same_thread_group(p->real_parent, group_leader)) {
+	real_parent = rcu_dereference(p->real_parent);
+	if (same_thread_group(real_parent, group_leader)) {
 		err = -EPERM;
 		if (task_session(p) != task_session(group_leader))
 			goto out;

base-commit: 6b8c8af514d739d0335f5579b585e02babe8a727
-- 
2.53.0