Dumping files

This is what CRIU does to dump information about opened files.

Files, descriptors and inodes in Linux

When a task opens a file Linux kernel constructs a chain of 3 objects to serve this.

Inode

This is an object which describes file as a couple of meta-data (owner, type, size) and data (the bytes themselves).

Dentry (directory entry)

A helper object that kernel uses to resolve file path into Inode object. If a file has hard-links, then one Inode will have several Dentries.

File

This one describes how a tasks works with an opened Dentry-Inode pair.

File descriptor

It's a number in task's table, that is used to reference the needed File object

So after open() (or some other, see below) system call there will be this chain in the memory

The File object can be referenced by more than on FDT, e.g. when a task calls fork() the child one gets new FDT, but it references the same Files as the parent does, i.e. Files become shared objects.

The Inode object is also interesting. First of all, remember that in Linux file descriptors can be obtained not only by the open() system call, but also by pipe() and socket() and a bunch of Linux-specific epoll_create, signalfd and others. So when serving this Linux would anyway create the mentioned above chain of File-Dentry-Inode objects, but the Inode one will be different for different calls. And CRIU knows this all and acts respectively :)

How info about opened files is stored in CRIU

Having said that CRIU introduces several images to keep info about what files are opened by task.

File descriptors

First image is the fdinfo-$id.img one. This image contains info about FDT of a process. The entries have two important fields -- fd and id.

The fd is the descriptor number under which the created on restore File should be put. The id is the identifier to the File-Inode object pair in other images.

Files themselves

Fore the sake of simplicity CRIU doesn't introduce separate states for Files, Dentries and Inodes leaving it to the kernel. Instead each this triplet is treated as one object and for every Inode type (file, pipe, socket, etc.) separate image is introduced. Thus CRIU has

• reg-files.img for regular files, that are created by open() call
• unixsk.img for unix sockets
• pipes.img for pipes
• inetsk for IP sockets (both TCP and UDP)
• signalfd.img for signal fd
• etc.

In each of this files info about File and Inode of respective file is preserved. Dentry information is effectively stored there for regular files only -- the file's path.

How CRIU gets the information to dump

So on dump CRIU needs to find out several things.

1. The FD numbers owned by tasks
2. How File-s are shared between tasks' FDTs
3. What Inode types are there
4. State of File and Inode objects

FD numbers

This is simple. Reading the /proc/$pid/fd or /proc/$pid/fdinfo directory just shows the required numbers.

Files sharing

In order to find out whether two Files sitting beyond two FDs of two tasks are the same CRIU uses the kcmp system call.

Determining Inode type

The inode type in most of the cases can be found out by stat()-ing the descriptor. To do this CRIU asks the parasite code to send back the files via unix socket's SCM_RIGHTS message. After this, having the same Files at hands CRIU fstats each and checks the st_mode field.

For some stupid files (signalfd, inotify, etc.) the mode field is zero and CRIU reads the /proc/$pid/fd/$fd/ link. For those the link target uniquely defines the Inode type.

State of File and Inode

From File CRIU needs only two things -- the mode and the position. Both can be read from /proc/$pid/fdinfo/$fd/ and the latter one can be requested via lseek call.

Getting the Inode state is specific to Inode. CRIU uses the following sources of information:

1. Data from /proc/$pid/fdinfo/$fd/
2. Link target of /proc/$pid/fd/$fd/ link
3. Inode-specific ioctl()-s
4. Fetch data directly from FD with recv + MSG_PEEK for socket queues and tee for pipes/fifos
5. Info from sock_diag modules to get info about sockets