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The criu utility dumps the state of processes/containers into a set of image files. This article describes the format of them.

Types of image files

CRIU images can be in one of the following formats

  • criu specific images in google protocol buffer format (PB format)
  • criu specific images with binary data in it
  • image files in 3rd party format (a.k.a. raw images)

Images in criu-specific format

All criu-specific image files begin with 2 32-bit magic cookies. The first cookie is the type of file (see below) the second is the optional sub-type of image. Images in PB format are followed by zero or more entries of the same type (not size!), each entry is preceded with 32-bit entry size value (not including this 32-bit value itself). Optionally each entry may be followed by extra payload which depends on the entry type.

Currently there are 3 types of images

Inventory file
This is the image file describing the set. It doesn't have sub-type magic.
Image file
Regular image. Most of the text below is about these files.
Auxiliary file
File that is not image, but criu generates one and it happens to be in protobuf format too. For now we have only stats and irmap cache files of that type. They also have sub-type magic.

IOW protocol-buffers image files look like

PAYLOAD    ::= "message encoded in ProtocolBuffer format"
EXTRA      ::= "arbitrary blob, depends on the PAYLOAD contents"

MAGIC      ::= "32 bit integer"
MAGIC_2    ::= "32 bit integer"
SIZE       ::= "32 bit integer, equals the PAYLOAD length"

Or, you can visualize it like

Type Size, bytes
Magic 4
Size0 4
Message0 Size0
... ...
SizeN 4
MessageN SizeN

The amount of entries in a image file depends on the type of file.

Images with PB data

Such images can be one of

Array image files
In these files the amount of entries can be any. You should read the image file up to the EOF to find out the exact number.
Single-entry image files
In these files exactly one entry is stored.

A file type can be guessed by the magic. The description of the entries in ProtocolBuffers language are in respective .proto files which reside in protobuf/ directory in the source tree.

name type description extra payload describing proto file
inventory single-entry Top level description of images - inventory.proto
fdinfo array Open file descriptors - fdinfo.proto
reg-files array Paths to files opened with open(2) syscall - regfile.proto
eventfd array Eventfd file information - eventfd.proto
eventpoll array Eventpoll file information - eventpoll.proto
eventpoll-tfd array Target file descriptors of eventpoll fds (merged into above) - eventpoll.proto
inotify array Inotify file information - intotify.proto
inotify-wd array Watch descriptors of inotify fds (merged into above) - inotify.proto
signalfd array signalfd info - signalfd.proto
core single-entry Core process info and (name, sigmask, itimers, etc.) arch-dependent information (registers, etc.) - core.proto
mm single-entry Address space information (VMAs, segments, exe file, etc.) - mm.proto
pipes array Pipes information - pipe.proto
pipes-data array Contents of pipes entry.bytes bytes of data sitting in a pipe pipe-data.proto
fifo array FIFO information - fifo.proto
fifo-data array Contents of FIFOs same as in pipes-data pipe-data.proto
pstree array Process tree linkage - pstree.proto
ids single IDs of objects (mm, files, sihand, etc.) and namespaces - core.proto
sigacts array Signal handling map - sa.proto
unixsk array Unix sockets - sk-unix.proto
inetsk array PF_INET sockets, both IPv4 and IPv6 - sk-inet.proto
sk-queues array Contents of socket queues entry.length bytes of data, one entry per packet sk-packet.proto
itimers array Interval timers state (merged into core image) - itimer.proto
creds single-entry Task credentials: uids, gids, caps, etc. - creds.proto
fs single-entry Chroot and chdir information - fs.proto
remap-fpath array File paths remaps (e.g. for invisible files) - remap-file-path.proto
ghost-file single-entry Ghost invisible files Right after the entry up to the EOF goes the contents of the file ghost-file.proto
tcp-stream single-entry TCP connection state (including data in queues) entry.inq_len bytes of in-queue data followed by entry.outq_len bytes of out-queue data tcp-stream.proto
mountpoints array Mountpoints information - mnt.proto
utsns single-entry Uname nodename and domainname of a UTS namespace - utsns.proto
tty array Information about opened TTYs - tty.proto
tty-info array Termios and similar stuff about TTYs - tty.proto
packetsk array Info about PF_PACKET sockets - packet-sock.proto
netdev array Info about network devices - netdev.proto

Images with memory dumps

Main article: memory dumps.

Anonymous memory contents (both private and shared) is stored in two types of images:

Pagemap files
These files contain info about which virtual regions are populated with data. The file is a set of protobuf messages.
Note.svg Note: Even though pagemap is an array kind of image (and can be included to the previous type), first pb message is of type pagemap_head and all the following ones are of type pagemap_entry.
Pages files
These contain 4k pages that are to be put into the memory according to the pagemap.

Raw images

These images contain data that were collected by criu with the help of some external tools.

Name Tool supporting the format Description
ifaddr ip from iproute2 IP addresses on network devices
route ip from iproute2 Routing tables
tmpfs tar + gzip Contents of a tmpfs filesystem

Notes about protobuf

We have a registered field number (1018) for custom options of all kinds. See protobuf/opts.proto for more info.

See also