[Devel] [PATCH RHEL7 COMMIT] fcntl: make F_GETOWN(EX) return 0 on dead owner task

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1160.11.1.vz7.172.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.11.1.vz7.172.11
------>
commit 5b05f716e35071368cc5c3b891c5a0527bee233c
Author: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Date:   Wed Feb 3 10:40:11 2021 +0300

    fcntl: make F_GETOWN(EX) return 0 on dead owner task
    
    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 b1be037..e72edd2 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;