[Devel] [PATCH RH7] fcntl: make F_GETOWN(EX) return 0 on dead owner task

[Pavel Tikhomirov]

Currently there is no way to differentiate file with alive owner from
file with dead owner but pid of the owner reused. That's why criu can't
actually know if it needs to restore file owner or not, because if it
restores owner but actual owner was dead, this can introduce unexpected
signals to the false-owner.

Let's change the api, so that F_GETOWN(EX) returns 0 in case actual
owner is dead already.

https://jira.sw.ru/browse/PSBM-124623

Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 fs/fcntl.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index b1be037ed2da9..e72edd20abe97 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -176,11 +176,15 @@ void f_delown(struct file *filp)
 
 pid_t f_getown(struct file *filp)
 {
-	pid_t pid;
+	pid_t pid = 0;
 	read_lock(&filp->f_owner.lock);
-	pid = pid_vnr(filp->f_owner.pid);
-	if (filp->f_owner.pid_type == PIDTYPE_PGID)
-		pid = -pid;
+	rcu_read_lock();
+	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type)) {
+		pid = pid_vnr(filp->f_owner.pid);
+		if (filp->f_owner.pid_type == PIDTYPE_PGID)
+			pid = -pid;
+	}
+	rcu_read_unlock();
 	read_unlock(&filp->f_owner.lock);
 	return pid;
 }
@@ -228,11 +232,14 @@ static int f_setown_ex(struct file *filp, unsigned long arg)
 static int f_getown_ex(struct file *filp, unsigned long arg)
 {
 	struct f_owner_ex __user *owner_p = (void __user *)arg;
-	struct f_owner_ex owner;
+	struct f_owner_ex owner = {};
 	int ret = 0;
 
 	read_lock(&filp->f_owner.lock);
-	owner.pid = pid_vnr(filp->f_owner.pid);
+	rcu_read_lock();
+	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type))
+		owner.pid = pid_vnr(filp->f_owner.pid);
+	rcu_read_unlock();
 	switch (filp->f_owner.pid_type) {
 	case PIDTYPE_MAX:
 		owner.type = F_OWNER_TID;
-- 
2.26.2