Release v4.3.0
What is MirageOS?
MirageOS is a library operating system that can build standalone unikernels on various platforms. More precisely, the architecture can be divided into:
- operating system libraries that implement kernel and protocol functionality, ranging from low-level network card drivers to a full reimplementation of the TLS protocol, through to a reimplementation of the Git protocol to store versioned data.
- A set of typed signatures to make sure these libraries are consistent and can interoperate. As all the library are almost all pure OCaml code, we have defined a set of OCaml module types that encode these conventions in a statically enforcable way. We make no compatibility guarantees at the C level, but compile those on a best-effort basis.
- Finally, MirageOS is also a metaprogramming compiler that generates OCaml code. It takes as input: the OCaml source code of a program and all of its dependencies, the full description of the deployment target, including configuration values (like the HTTP port to listen on, or the private key or the service being deployed). The `mirage`CLI tool uses all of these to generate a executable unikernel: a specialised binary artefact containing only the code what is needed to run on the given deployment platform and no more.
It is possible to write high-level MirageOS applications, such as HTTPS, email or CalDAV servers which can be deployed on very heterogenous and embedded platforms by changing only a few compilation parameters. The supported platforms range from minimal virtual machines running on cloud providers, or processes running inside Docker containers configured with a tight security profile. In general, these platform do not have a full POSIX environment; MirageOS does not try to emulate POSIX and focuses on providing a small, well-defined, typed interface with the system components. The nearest equivalent to the MirageOS approach is the WASI (wasi.dev) set of interfaces for WebAssembly.
Is everything really written in OCaml?
While most of the code is written in OCaml, a typed, high-level language with many good safety properties, there are pieces of MirageOS which are still written in C. These bits can be separated in three categories:
- The OCaml runtime is written in C. It needs to be ported to the platform that MirageOS is trying to target, which do not support POSIX. Hence, the first component to port to a new platform is the OCaml runtime.
- The low-level device drivers (network, console, clock, etc) also need some C bits.
- The base usual C bindings; some libraries are widely used and (unfortunately) very hard (but not impossible) to replace them completely without taking a big performance hit or having to trust code without much real-world usages. This is the case for low-level bit handling for crypto code (even if we try to make sure allocation is alway handled by the OCaml runtime) as well as arbitrary precision numeric computation (e.g. gmp). Ideally we could image rewriting all of these libraries in OCaml if we had an infinite amount of time in our hands.
MirageOS as a cross-compilator
The MirageOS compiler is basically a cross-compiler, where the host and target toolchain are identical, but with different flags for the C bindings: for instance, it is necessary to pass -freestanding to all C bindings to not use POSIX headers. The MirageOS compiler also uses a custom linker: eg. not only it needs a custom OCaml's runtime libasmrun.a, but it also needs to run a different linker to generate specialised executable images.
Historically, the OCaml ecosystem always had partial support for cross-compilation: for instance, the ocaml-cross way of doing it is to duplicate all existing opam pacakges by adding a -windows suffix to their names and dependencies; this allows normal packages and windows packages can be co-installed in the same opam switch.
MirageOS 3.x
MirageOS 3.x solves this by duplicating only the packages defining C bindings. It relies on every MirageOS backend registering a set of CFLAGS with pkg-config. Then every bindings uses pkg-config to configure their CFLAGS and ocamlfind to register link-time predicates, e.g. additional link time options like the name of the C archives. Finally, the final link step is done by querying ocamlfind (using the custom registered predicates) to link the list of dependencies' objects files with the result of OCam compiler's --output-obj option.
MirageOS 4.x
MirageOS 4 solves this by relying on dune's built-in support for cross-compilation. This is done by gathering all the sources of the dependencies locally with opam-monorepo, and by creating a `dune-workspace` file describing the C flags to use in each cross-compilation "context". Once this is set-up, only one dune build can cross-compile the unikernel target with all its local sources.
MirageOS eDSL
The rest of the document describes Functoria, the embedded domain-specific language to be used in config.ml files, to described how the typed libraries have to be assembled.
Combinators
The type for values representing module types.
type t is a value representing the module type t.
val (@->) : 'a typ -> 'b typ -> ('a -> 'b) typConstruct a functor type from a type and an existing functor type. This corresponds to prepending a parameter to the list of functor parameters. For example:
kv_ro @-> ip @-> kv_ro
This describes a functor type that accepts two arguments -- a kv_ro and an ip device -- and returns a kv_ro.
The type for values representing module implementations.
m $ a applies the functor m to the module a.
Same as impl but with hidden type.
dep t is the (build-time) dependency towards t.
Keys
The type for command-line parameters.
The type for abstract keys.
The type for values parsed from the command-line. See Key.value.
key k is an untyped representation of k.
if_impl v impl1 impl2 is impl1 if v is resolved to true and impl2 otherwise.
match_impl v cases ~default chooses the implementation amongst cases by matching the v's value. default is chosen if no value matches.
Package dependencies
For specifying opam package dependencies, the type package is used. It consists of the opam package name, the ocamlfind names, and optional lower and upper bounds. The version constraints are merged with other modules.
The type for opam packages.
Installation scope of a package.
val package :
?scope:scope ->
?build:bool ->
?sublibs:string list ->
?libs:string list ->
?min:string ->
?max:string ->
?pin:string ->
?pin_version:string ->
string ->
packageApplication Builder
Values of type impl are tied to concrete module implementation with the device and foreign construct. Module implementations of type job can then be registered into an application builder. The builder is in charge if parsing the command-line arguments and of generating code for the final application. See Functoria.Lib for details.
The type for build information.
Alias for main, where ?extra_deps has been renamed to ?deps.
val main :
?packages:package list ->
?packages_v:package list value ->
?keys:abstract_key list ->
?extra_deps:abstract_impl list ->
string ->
'a typ ->
'a implforeign name typ is the functor name, having the module type typ. The connect code will call <name>.start.
- If
packages or packages_v is set, then the given packages are installed before compiling the current application. - If
keys is set, use the given keys to parse at configure and runtime the command-line arguments before calling <name>.connect. - If
extra_deps is set, the given list of abstract implementations is added as data-dependencies: they will be initialized before calling <name>.connect.
Devices
of_device t is the implementation device t.
impl ... is of_device @@ Device.v ...
Jobs
General mirage devices
Implementation of the tracing type.
Use mirage-profile to trace the unikernel. On Unix, this creates and mmaps a file called "trace.ctf". On Xen, it shares the trace buffer with dom0.
For the Qubes target, the Qubes database from which to look up * dynamic runtime configuration information.
A default qubes database, guessed from the usual valid configurations.
Time
Abstract type for timers.
Implementations of the Mirage_time.S signature.
The default timer implementation.
Clocks
Abstract type for POSIX clocks.
Implementations of the Mirage_clock.PCLOCK signature.
The default mirage-clock PCLOCK implementation.
Abstract type for monotonic clocks
Implementations of the Mirage_clock.MCLOCK signature.
The default mirage-clock MCLOCK implementation.
Log reporters
The type for log reporters.
Implementation of the log reporter type.
default_reporter ?clock ?level () is the log reporter that prints log messages to the console, timestampted with clock. If not provided, the default clock is default_posix_clock. level is the default log threshold. It is Logs.Info if not specified.
no_reporter disable log reporting.
Random
Abstract type for random sources.
Implementations of the Mirage_random.S signature.
Default PRNG device to be used in unikernels. It uses getrandom/getentropy on Unix, and a Fortuna PRNG on other targets.
rng is the device Mirage_crypto_rng.Make.
Consoles
Abstract type for consoles.
Implementations of the Mirage_console.S signature.
Default console implementation.
Custom console implementation.
Block devices
Abstract type for raw block device configurations.
Implementations of the Mirage_block.S signature.
Use the given file as a raw block device.
val block_of_xenstore_id : string -> block implUse the given XenStore ID (ex: /dev/xvdi1 or 51760) as a raw block device.
Use a ramdisk with the given name.
val generic_block :
?group:string ->
?key:[ `XenstoreId | `BlockFile | `Ramdisk ] value ->
string ->
block implStatic key/value stores
Abstract type for read-only key/value store.
Implementations of the Mirage_kv.RO signature.
Direct access to the underlying filesystem as a key/value store. For Xen backends, this is equivalent to crunch.
Use a FAT formatted block device.
val generic_kv_ro :
?group:string ->
?key:[ `Crunch | `Direct ] value ->
string ->
kv_ro implGeneric key/value that will choose dynamically between direct_kv_ro and crunch. To use a filesystem implementation, try kv_ro_of_fs.
If no key is provided, it uses Key.kv_ro to create a new one.
val docteur :
?mode:[ `Fast | `Light ] ->
?disk:string Key.key ->
?analyze:bool Key.key ->
?branch:string ->
?extra_deps:string list ->
string ->
kv_ro impldocteur ?mode ?disk ?analyze remote is a read-only, key-value store device. Data is stored on that device using the Git PACK file format, version 2. This format has very good compression factors for many similar files of relatively small size. For instance, 14Gb of HTML files can be compressed into a disk image of 240Mb.
Unlike crunch, docteur produces an external image which means that less memory is used to keep and get files. The image can be produced from many sources:
- A local Git repository (like
file://path/to/the/git/repository/) - A simple directory (like
file://path/to/a/simple/directory/) - A remote Git repository (via SSH, HTTP(S) or TCP/IP as what
git clone expects)
If you use a Git repository, you can choose a specific branch with the ?branch argument (like refs/heads/main). Otherwise, this argument is ignored.
If you use a simple directory, it can be a relative from your unikernel project (relativize://directory) or an absolute path (file://home/user/directory).
If a required file is produced by a dune rule, you must notice it via the extra_deps argument.
For a Solo5 target, users must attach the image as a block device:
$ solo5-hvt --block:<name>=<path-to-the-image> -- unikernel.{hvt,...}
For the Unix target, the program open the image at the beginning of the process. An integrity check of the image can be done via the analyze value (defaults to true).
It's possible to use the file-system into 2 modes:
`Light: any access requires that we reconstruct the path to the requested file. That means that we will need to extract a few additional objects before the extraction of the requested one. `Light does not cache anything in memory but it can be slower if the requested file is deep in the directory structure.`Fast: reconstructs and cache the layout of the directory structure when the unikernel starts: it might increase boot-time and bigger memory requirements. However, `Fast allows the device to decode only the requested object so it is faster than the `Light mode.
Abstract type for read-write key/value store.
Implementations of the Mirage_kv.RW signature.
Direct access to the underlying filesystem as a key/value store. Only available on Unix backends.
An in-memory key-value store using mirage-kv-mem.
chamelon ~program_block_size returns a kv_rw filesystem which is an implementation of littlefs in OCaml. The chamelon device expects a block-device:
let program_block_size =
let doc = Key.Arg.info [ "program-block-size" ] in
Key.(create "program_block_size" Arg.(opt int 16 doc))
let block = block_of_file "db"
let fs = chamelon ~program_block_size block
For Solo5 targets, you finally can launch the unikernel with:
$ solo5-hvt --block:db=db.img unikernel.hvt
The block-device must be well-formed and formatted by the chamelon tool:
$ dd if=/dev/zero if=db.img bs=1M count=1
$ chamelon format db.img 512
Network interfaces
Abstract type for network configurations.
Implementations of the Mirage_net.S signature.
default_network is a dynamic network implementation * which attempts to do something reasonable based on the target.
Ethernet configuration
Implementations of the Ethernet.S signature.
ARP configuration
Implementation of the Arp.S signature.
ARP implementation provided by the arp library
IP configuration
Implementations of the Tcpip.Ip.S signature.
Abstract type for IP configurations.
The Tcpip.Ip.S module signature with ipaddr = Ipaddr.V4.
The Tcpip.Ip.S module signature with ipaddr = Ipaddr.V6.
The Tcpip.Ip.S module signature with ipaddr = Ipaddr.t.
type ipv4_config = {network : Ipaddr.V4.Prefix.t;gateway : Ipaddr.V4.t option;
}Types for manual IPv4 configuration.
type ipv6_config = {network : Ipaddr.V6.Prefix.t;gateway : Ipaddr.V6.t option;
}Types for manual IPv6 configuration.
Use an IPv4 address Exposes the keys Key.V4.network and Key.V4.gateway. If provided, the values of these keys will override those supplied in the ipv4 configuration record, if that has been provided.
Use a given initialized QubesDB to look up and configure the appropriate * IPv4 interface.
UDP configuration
Implementation of the Tcpip.Udp.S signature.
val socket_udpv4v6 :
?group:string ->
Ipaddr.V4.t option ->
Ipaddr.V6.t option ->
udpv4v6 implTCP configuration
Implementation of the Tcpip.Tcp.S signature.
val socket_tcpv4v6 :
?group:string ->
Ipaddr.V4.t option ->
Ipaddr.V6.t option ->
tcpv4v6 implNetwork stack configuration
Dual IPv4 and IPv6
Implementation of the Tcpip.Stack.V4V6 signature.
Direct network stack with given ip.
Network stack with sockets.
Build a stackv4v6 by checking the Key.V6.network, and Key.V6.gateway keys for IPv4 and IPv6 configuration information, filling in unspecified information from ?config, then building a stack on top of that.
Generic stack using a net keys: Key.net.
If a key is not provided, it uses Key.net (with the group argument) to create it.
tcpv4v6 stackv4v6 is an helper to extract the TCP/IP stack regardless the UDP/IP stack expected by some devices such as protocols.
Resolver configuration
DNS client
A DNS client is a module which implements:
getaddrinfo to request a query_type-dependent response to a nameserver regarding a domain-name such as the MX record.gethostbyname to request the A regarding a domain-namegethostbyname6 to request the AAAA record regarding a domain-name
generic_dns_client stackv4v6 creates a new DNS value which is able to resolve domain-name from nameservers. It requires a network stack to communicate with these nameservers.
Happy-eyeballs
Happy-eyeballs is an implementation of RFC 8305 which specifies how to connect to a remote host using either IP protocol version 4 or IP protocol version 6 from a stackv4v6 network implementation.
The given device is able to resolve a remote host via a dns_client device and both must share the same stackv4v6 implementation.
generic_happy_eyeballs stackv4v6 dns_client creates a new happy-eyeballs value which is able to resolve and connect to a remote host and allocate finally a connected flow from the given network implementation stackv4v6.
This device has several optional arguments of keys for timeouts specified in nanoseconds.
Syslog configuration
Syslog exfiltrates log messages (generated by libraries using the logs library) via a network connection. The log level of the log sources is controlled via the Mirage_key.logs key. The functionality is provided by the logs-syslog package.
type syslog_config = {hostname : string;server : Ipaddr.t option;port : int option;truncate : int option;
}val syslog_config :
?port:int ->
?truncate:int ->
?server:Ipaddr.t ->
string ->
syslog_configImplementation of the syslog type.
Emit log messages via UDP to the configured host.
Emit log messages via TCP to the configured host.
Emit log messages via TLS to the configured host, using the credentials (private key, certificate, trust anchor) provided in the KV_RO using the keyname.
Conduit configuration
HTTP configuration
cohttp_server starts a Cohttp server.
httpaf_server starts a http/af server.
cohttp_server starts a Cohttp server.
Argv configuration
default_argv is a dynamic argv implementation * which attempts to do something reasonable based on the target.
no_argv Disable command line parsing and set argv to |""|.
Git client configuration
Users can connect to a remote Git repository in many ways:
The devices defined below define these in composable ways. The git_client impl returned from them can be passed to Git or Irmin in order to be able to fetch and push from/into a Git repository.
The user is able to restrict or enlarge protocol possibilities needed for its application. For instance, the user is able to restrict only the SSH connection to communicate with a Git repository or the user can handle TCP/IP and SSH as possible protocols to communicate with a peer.
For instance, a device which is able to communicate via TCP/IP and SSH can be implemented like:
let git_client =
let dns = happy_eyeballs stackv4v6 in
let ssh = git_ssh ~key (tcpv4v6_of_stackv4v6 stackv4v6) dns in
let tcp = git_tcp (tcpv4v6_of_stackv4v6 stackv4v6) dns in
merge_git_clients ssh tcp
The type for devices that implement the Git protocol.
merge_git_clients a b is a device that can connect to remote Git repositories using either the device a or the device b.
git_tcp tcpv4v6 dns is a device able to connect to a remote Git repository using TCP/IP.
git_ssh ?authenticator ~key tcpv4v6 dns is a device able to connect to a remote Git repository using an SSH connection with the given private key. The identity of the remote Git repository can be verified using authenticator.
The format of the private key is: <type>:<seed or b64 encoded>. <type> can be rsa or ed25519 and, if the type is RSA, we expect the seed of the private key. Otherwise (if the type is Ed25519), we expect the b64-encoded private key.
The format of the authenticator is SHA256:<b64-encoded-public-key>, the output of:
$ ssh-keygen -lf <(ssh-keyscan -t rsa|ed25519 remote 2>/dev/null)
git_http ?authenticator ?headers tcpv4v6 dns is a device able to connect to a remote Git repository via an HTTP(S) connection, using the provided HTTP headers. The identity of the remote Git repository can be verified using authenticator.
The format of it is:
none no authentication- key(:<hash>)?:<b64-encoded fingerprint> to authenticate via the key fingerprint
- cert(:<hash>)?:<b64-encoded fingerprint> to authenticate via the cert fingerprint
- trust-anchor(:<der-encoded cert>)+ to authenticate via a list of certificates - By default, we use X.509 trust anchors extracted from Mozilla's NSS
Other devices
job is the combinator for representing main tasks.
noop is a job that does nothing, has no dependency and returns ()
keys argv is a job that loads argv.
info is the combinator to generate info values to use at runtime.
app_info exports all the information available at configure time into a runtime Mirage.Info.t value.
val app_info_with_opam_deps : (string * string) list -> info implapp_info exports all the information available at configure time into a runtime Mirage.Info.t value.
Application registering
register name jobs registers the application named by name which will executes the given jobs.