Aleksa Sarai
e377e16846
We will almost certainly need to eventually rework nsenter to:
1. Figure out a way to make pthread_self() not break after nsenter runs
(probably not possible, because the core issue is likely that we are
ignoring the rules of signal-safety(7)); or
2. Do an other re-exec of /proc/self/exe to execute the Go half of
"runc init" -- after we've done the nsenter setup. This would reset
all of the process state and ensure we have a clean glibc state for
Go, but it would make runc slower...
For now, just block Go 1.22 builds to avoid having broken runcs floating
around until we resolve the issue. It seems possible for musl to also
have an issue, but it appears to work and so for now just block glibc
builds.
Note that this will only block builds for anything that uses nsenter --
so users of our (internal) libcontainer libraries should be fine. Only
users that are starting containers using nsenter to actually start
containers will see the error (which is precisely what we want).
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
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.