[PATCH v2] pid: restore the old behaviour of the ns_last_pid sysctl

[Andrei Vagin]

CRIU uses ns_last_pid to fork a process with a specified pid. For
example, if we need to create a process with the pid of 10000,
we write 9999 into /proc/sys/kernel/ns_last_pid

$ echo 9999 > /proc/sys/kernel/ns_last_pid; sh -c 'echo $$'
10000

This behaviour has been broken and now if we write 9999 to ns_last_pid,
a process will get the pid 9999. This patch restores the old behaviour.

v2: make code a bit more readable // Oleg

fixes: ("pid: replace pid bitmap implementation with IDR API")
Cc: Gargi Sharma <gs051095@xxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>
---
 kernel/pid_namespace.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index fea2c24fa460..0b53eef7d34b 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -287,6 +287,7 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
 {
 	struct pid_namespace *pid_ns = task_active_pid_ns(current);
 	struct ctl_table tmp = *table;
+	int ret, next;
 
 	if (write && !ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
@@ -297,8 +298,14 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
 	 * it should synchronize its usage with external means.
 	 */
 
-	tmp.data = &pid_ns->idr.idr_next;
-	return proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+	next = idr_get_cursor(&pid_ns->idr) - 1;
+
+	tmp.data = &next;
+	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+	if (!ret && write)
+		idr_set_cursor(&pid_ns->idr, next + 1);
+
+	return ret;
 }
 
 extern int pid_max;
-- 
2.13.6