nsenter package registers a special init constructor that is called before
the Go runtime has a chance to boot. This provides us the ability to
existing namespaces and avoid the issues that the Go runtime has with multiple
threads. This constructor will be called if this package is registered,
imported, in your go application.
nsenter package will
import "C" and it uses cgo
package. In cgo, if the import of "C" is immediately preceded by a comment, that comment,
called the preamble, is used as a header when compiling the C parts of the package.
So every time we import package
nsenter, the C code function
nsexec() would be
called. And package
nsenter is only imported in
init.go, so every time the runc
init command is invoked, that C code is run.
nsexec() must be run before the Go runtime in order to use the
Linux kernel namespace, you must
import this library into a package if
you plan to use
libcontainer directly. Otherwise Go will not execute
nsexec() constructor, which means that the re-exec will not cause
the namespaces to be joined. You can import it like this:
import _ "github.com/opencontainers/runc/libcontainer/nsenter"
nsexec() will first get the file descriptor number for the init pipe
from the environment variable
_LIBCONTAINER_INITPIPE (which was opened
by the parent and kept open across the fork-exec of the
process). The init pipe is used to read bootstrap data (namespace paths,
clone flags, uid and gid mappings, and the console path) from the parent
nsexec() will then call
setns(2) to join the namespaces
provided in the bootstrap data (if available),
clone(2) a child process
with the provided clone flags, update the user and group ID mappings, do
some further miscellaneous setup steps, and then send the PID of the
child process to the parent of the
nsexec() "caller". Finally,
nsexec() will exit and the child
nsexec() process will
return to allow the Go runtime take over.
NOTE: We do both
clone(2) even if we don't have any
CLONE_NEW* clone flags because we must fork a new process in order to
enter the PID namespace.