[PATCH 0/2] fs/lock: show locks info owned by dead/invisible processes

[Konstantin Khorenko]

The behavior has been changed after 9d5b86ac13c5 ("fs/locks: Remove fl_nspid
and use fs-specific l_pid for remote locks")
and now /proc/$PID/fdinfo/$FD does not show the info about the lock
* if the flock owner process is dead and its pid has been already freed
or
* if the lock owner is not visible in current pidns.

CRIU uses this interface to store locks info during dump and thus can break
on v4.13 and newer.

So let's show info about locks anyway in described cases (like it was before
9d5b86ac13c5), but show pid number saved in file_lock struct if we are in
init_pid_ns (patch 1) or just zero otherwise (patch 2) like we do with SID.

Reproducer:
process A       process A1      process A2
fork()--------->
exit()          open()
                flock()
                fork()--------->
                exit()          sleep()

Before the patch:
================
(root@vz7)/: cat /proc/${PID_A2}/fdinfo/3
pos:    4
flags:  02100002
mnt_id: 257
lock:   (root@vz7)/:

After the patch:
===============
(root@vz7)/:cat /proc/${PID_A2}/fdinfo/3
pos:    4
flags:  02100002
mnt_id: 295
lock:   1: FLOCK  ADVISORY  WRITE ${PID_A1} b6:f8a61:529946 0 EOF

===============
# cat flock1.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <fcntl.h>

int main(void)
{
        int fd;
        int err;
        pid_t child_pid;

        child_pid = fork();
        if (child_pid == -1)
                perror("fork failed");
        if (child_pid) {
                exit(0);
        }

        fd = open("/tmp/a", O_CREAT | O_RDWR);
        if (fd == -1)
                perror("Failed to open the file");

        err = flock(fd, LOCK_EX);
        if (err == -1)
                perror("flock failed");

        child_pid = fork();
        if (child_pid == -1)
                perror("fork failed");
        if (child_pid)
                exit(0);

        sleep(10000);

        return 0;
}

Konstantin Khorenko (2):
  fs/lock: skip lock owner pid translation in case we are in init_pid_ns
  fs/lock: show locks taken by processes from another pidns

 fs/locks.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

-- 
2.15.1