6bd4bd9030
Effectively, !dumpable makes implementing rootless containers quite hard, due to a bunch of different operations on /proc/self no longer being possible without reordering everything. !dumpable only really makes sense when you are switching between different security contexts, which is only the case when we are joining namespaces. Unfortunately this means that !dumpable will still have issues in this instance, and it should only be necessary to set !dumpable if we are not joining USER namespaces (new kernels have protections that make !dumpable no longer necessary). But that's a topic for another time. This also includes code to unset and then re-set dumpable when doing the USER namespace mappings. This should also be safe because in principle processes in a container can't see us until after we fork into the PID namespace (which happens after the user mapping). In rootless containers, it is not possible to set a non-dumpable process's /proc/self/oom_score_adj (it's owned by root and thus not writeable). Thus, it needs to be set inside nsexec before we set ourselves as non-dumpable. Signed-off-by: Aleksa Sarai <asarai@suse.de> |
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|---|---|---|
| .. | ||
| README.md | ||
| 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 now only imported in main_unix.go, so every time
before we call cmd.Start on linux, that C code would 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.