16612d74de
The usage of memfd_create(2) and other copying techniques is quite wasteful, despite attempts to minimise it with _LIBCONTAINER_STATEDIR. memfd_create(2) added ~10M of memory usage to the cgroup associated with the container, which can result in some setups getting OOM'd (or just hogging the hosts' memory when you have lots of created-but-not-started containers sticking around). The easiest way of solving this is by creating a read-only bind-mount of the binary, opening that read-only bindmount, and then umounting it to ensure that the host won't accidentally be re-mounted read-write. This avoids all copying and cleans up naturally like the other techniques used. Unfortunately, like the O_TMPFILE fallback, this requires being able to create a file inside _LIBCONTAINER_STATEDIR (since bind-mounting over the most obvious path -- /proc/self/exe -- is a *very bad idea*). Unfortunately detecting this isn't fool-proof -- on a system with a read-only root filesystem (that might become read-write during "runc init" execution), we cannot tell whether we have already done an ro remount. As a partial mitigation, we store a _LIBCONTAINER_CLONED_BINARY environment variable which is checked *alongside* the protection being present. Signed-off-by: Aleksa Sarai <asarai@suse.de> |
||
|---|---|---|
| .. | ||
| README.md | ||
| cloned_binary.c | ||
| namespace.h | ||
| nsenter.go | ||
| nsenter_gccgo.go | ||
| nsenter_test.go | ||
| nsenter_unsupported.go | ||
| nsexec.c | ||
README.md
nsenter
The 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 setns on
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.
The 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.
Because 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
the 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 nsexec() init
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
process. 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,
the parent nsexec() will exit and the child nsexec() process will
return to allow the Go runtime take over.
NOTE: We do both setns(2) and 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.