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diff --git a/g3doc/architecture_guide/platforms.md b/g3doc/architecture_guide/platforms.md new file mode 100644 index 000000000..6e63da8ce --- /dev/null +++ b/g3doc/architecture_guide/platforms.md @@ -0,0 +1,86 @@ +# Platform Guide + +A gVisor sandbox consists of multiple processes when running. These processes +collectively comprise a shared environment in which one or more containers can +be run. + +Each sandbox has its own isolated instance of: + +* The **Sentry**, A user-space kernel that runs the container and intercepts + and responds to system calls made by the application. + +Each container running in the sandbox has its own isolated instance of: + +* A **Gofer** which provides file system access to the container. + + + +## runsc + +The entrypoint to running a sandboxed container is the `runsc` executable. +`runsc` implements the [Open Container Initiative (OCI)][oci] runtime +specification. This means that OCI compatible _filesystem bundles_ can be run by +`runsc`. Filesystem bundles are comprised of a `config.json` file containing +container configuration, and a root filesystem for the container. Please see the +[OCI runtime spec][runtime-spec] for more information on filesystem bundles. +`runsc` implements multiple commands that perform various functions such as +starting, stopping, listing, and querying the status of containers. + +## Sentry + +The Sentry is the largest component of gVisor. It can be thought of as a +userspace OS kernel. The Sentry implements all the kernel functionality needed +by the untrusted application. It implements all of the supported system calls, +signal delivery, memory management and page faulting logic, the threading model, +and more. + +When the untrusted application makes a system call, the currently used platform +redirects the call to the Sentry, which will do the necessary work to service +it. It is important to note that the Sentry will not simply pass through system +calls to the host kernel. As a userspace application, the Sentry will make some +host system calls to support its operation, but it will not allow the +application to directly control the system calls it makes. + +The Sentry aims to present an equivalent environment to (upstream) Linux v4.4. + +File system operations that extend beyond the sandbox (not internal /proc files, +pipes, etc) are sent to the Gofer, described below. + +## Platforms + +gVisor requires a platform to implement interception of syscalls, basic context +switching, and memory mapping functionality. + +### ptrace + +The ptrace platform uses `PTRACE_SYSEMU` to execute user code without allowing +it to execute host system calls. This platform can run anywhere that ptrace +works (even VMs without nested virtualization). + +### KVM (experimental) + +The KVM platform allows the Sentry to act as both guest OS and VMM, switching +back and forth between the two worlds seamlessly. The KVM platform can run on +bare-metal or in a VM with nested virtualization enabled. While there is no +virtualized hardware layer -- the sandbox retains a process model -- gVisor +leverages virtualization extensions available on modern processors in order to +improve isolation and performance of address space switches. + +## Gofer + +The Gofer is a normal host Linux process. The Gofer is started with each sandbox +and connected to the Sentry. The Sentry process is started in a restricted +seccomp container without access to file system resources. The Gofer provides +the Sentry access to file system resources via the 9P protocol and provides an +additional level of isolation. + +## Application + +The application (aka the untrusted application) is a normal Linux binary +provided to gVisor in an OCI runtime bundle. gVisor aims to provide an +environment equivalent to Linux v4.4, so applications should be able to run +unmodified. However, gVisor does not presently implement every system call, +/proc file, or /sys file so some incompatibilities may occur. + +[oci]: https://www.opencontainers.org +[runtime-spec]: https://github.com/opencontainers/runtime-spec |