Pidfd

This article describes how we checkpoint/restore pidfds (process file descriptors).

Checkpointing

All information that we require for restore of pidfds is available in this /proc entry: /proc/$pid/fdinfo/$pidfd

Since CRIU does not support nested pid namespaces, the correct pid to use during restore is the last pid (i.e., pid in the most deeply nested pid namespace) in the /proc/$pid/fdinfo/$pidfd/NSpid field. So, we only dump that pid.

Restoring pidfd(s) Pointing to an Alive Process

CRIU, while restoring, first creates all processes in the process tree and then starts opening file descriptors in each process.

So, If the pidfd points to an alive process, we can simply use pidfd_open() to create the pidfd.

Restoring pidfd(s) Pointing to a Dead Process

If the process (to which the pidfd was pointing) is dead, however, we lose access to its pid (/proc/$pid/fdinfo/$pidfd/NSpid becomes -1). We also cannot open a pidfd pointing to a dead process.

To overcome these problems, we do the following things:

We create a hashtable with the inode number of the pidfd acting as the key that stores:

• List of all processes that had a pidfd open with this inode number.
• The highest id in the list of processes (creator_id)

For each unique inode number, the process with creator_id (let's call it creator) creates a temporary process (let's name this process x).

The creator process will then open pidfd(s) pointing to x and will send them to all other processes (that had a pidfd with this inode number) using CRIU's send_desc_to_peer and recv_desc_from_peer functions, which allow processes to send and receive file descriptors.

This way, all processes with pidfds that point to the same dead process remain in the correct state (i.e., have pidfds with the same inode number) even after restoration.

After the creator process finishes opening and sending all pidfds, it kills the temporary process x.

Limitations

• We currently do not support pidfd's opened with the PIDFD_THREAD flag.
• We also cannot C/R pidfd's that point to process(es) outside the current process tree.