on 2012/11/17 00:35, Eric W. Biederman wrote: > From: "Eric W. Biederman" <[email protected]> > > Unsharing of the pid namespace unlike unsharing of other namespaces > does not take affect immediately. Instead it affects the children > created with fork and clone. The first of these children becomes the init > process of the new pid namespace, the rest become oddball children > of pid 0. From the point of view of the new pid namespace the process > that created it is pid 0, as it's pid does not map. > > A couple of different semantics were considered but this one was > settled on because it is easy to implement and it is usable from > pam modules. The core reasons for the existence of unshare. > > I took a survey of the callers of pam modules and the following > appears to be a representative sample of their logic. > { > setup stuff include pam > child = fork(); > if (!child) { > setuid() > exec /bin/bash > } > waitpid(child); > > pam and other cleanup > } > > As you can see there is a fork to create the unprivileged user > space process. Which means that the unprivileged user space > process will appear as pid 1 in the new pid namespace. Further > most login processes do not cope with extraneous children which > means shifting the duty of reaping extraneous child process to > the creator of those extraneous children makes the system more > comprehensible. > > The practical reason for this set of pid namespace semantics is > that it is simple to implement and verify they work correctly. > Whereas an implementation that requres changing the struct > pid on a process comes with a lot more races and pain. Not > the least of which is that glibc caches getpid(). > > These semantics are implemented by having two notions > of the pid namespace of a proces. There is task_active_pid_ns > which is the pid namspace the process was created with > and the pid namespace that all pids are presented to > that process in. The task_active_pid_ns is stored > in the struct pid of the task. > > Then there is the pid namespace that will be used for children > that pid namespace is stored in task->nsproxy->pid_ns. > > Signed-off-by: Eric W. Biederman <[email protected]> > ---
Acked-by: Gao feng <[email protected]> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
