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 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 32-bit magic cookie. 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.
IOW protocol-buffers image files look like
IMAGE_FILE ::= MAGIC { ENTRY }
ENTRY ::= SIZE PAYLOAD [ EXTRA ]
PAYLOAD ::= "message encoded in ProtocolBuffer format"
EXTRA ::= "arbitrary blob, depends on the PAYLOAD contents"
MAGIC ::= "32 bit integer"
SIZE ::= "32 bit integer, equals the PAYLOAD length"
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 | - | eventpoll.proto |
| inotify | array | Inotify file information | - | intotify.proto |
| inotify-wd | array | Watch descriptors of inotify fds | - | inotify.proto |
| signalfd | array | signalfd info | - | signalfd.proto |
| core | single-entry | Arch-dependent information (registers, etc.) | - | core.proto |
| mm | single-entry | Address space generic information (segments, exe file, etc.) | - | mm.proto |
| vmas | array | Virtual mappings (mmap(2)) |
- | vma.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 and IDs | - | pstree.proto |
| sigacts | array | Signal handling map | - | sa.proto |
| unixsk | array | Unix domain 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 | - | 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 | - | remap-file-path.proto |
| ghost-file | single-entry | Ghost files (those, not visible from FS, but still used by tasks) | 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) | - | 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 tty-s | - | tty.proto |
| tty-info | array | Termios and similar stuff about tty-s | - | tty.proto |
| packetsk | array | Info about PF_PACKET sockets | - | packet-sock.proto |
| netdev | array | Info about network devices | - | netdev.proto |
Images with binary data
These are
- Memory dumps
- These files contain memory dumps. The format is
IMAGE ::= MAGIC [ ADDRESS DATA ] ADDRESS ::= "64-bit virtual address" DATA ::= "4K bytes blob (i.e. -- memory page)"
Raw images
These images contain data that were collected by criu with the help of some external tools. These are
| name | tool that understand this format | description |
|---|---|---|
| ifaddr | iproute2's ip | Contains info about IP addresses on network devices |
| route | iproute2's ip | Contains routing tables |
| tmpfs | tar + gzip | Contains contents of tmpfs filesystem |