On Mon, Sep 15, 2025 at 03:25:20PM +0200, Jan Kara wrote: > On Fri 12-09-25 13:52:51, Christian Brauner wrote: > > A while ago we added support for file handles to pidfs so pidfds can be > > encoded and decoded as file handles. Userspace has adopted this quickly > > and it's proven very useful. Implement file handles for namespaces as > > well. > > > > A process is not always able to open /proc/self/ns/. That requires > > procfs to be mounted and for /proc/self/ or /proc/self/ns/ to not be > > overmounted. However, userspace can always derive a namespace fd from > > a pidfd. And that always works for a task's own namespace. > > > > There's no need to introduce unnecessary behavioral differences between > > /proc/self/ns/ fds, pidfd-derived namespace fds, and file-handle-derived > > namespace fds. So namespace file handles are always decodable if the > > caller is located in the namespace the file handle refers to. > > > > This also allows a task to e.g., store a set of file handles to its > > namespaces in a file on-disk so it can verify when it gets rexeced that > > they're still valid and so on. This is akin to the pidfd use-case. > > > > Or just plainly for namespace comparison reasons where a file handle to > > the task's own namespace can be easily compared against others. > > > > Reviewed-by: Amir Goldstein <[email protected]> > > Signed-off-by: Christian Brauner <[email protected]> > > ... > > > + switch (ns->ops->type) { > > +#ifdef CONFIG_CGROUPS > > + case CLONE_NEWCGROUP: > > + if (!current_in_namespace(to_cg_ns(ns))) > > + owning_ns = to_cg_ns(ns)->user_ns; > > + break; > > +#endif > > +#ifdef CONFIG_IPC_NS > > + case CLONE_NEWIPC: > > + if (!current_in_namespace(to_ipc_ns(ns))) > > + owning_ns = to_ipc_ns(ns)->user_ns; > > + break; > > +#endif > > + case CLONE_NEWNS: > > + if (!current_in_namespace(to_mnt_ns(ns))) > > + owning_ns = to_mnt_ns(ns)->user_ns; > > + break; > > +#ifdef CONFIG_NET_NS > > + case CLONE_NEWNET: > > + if (!current_in_namespace(to_net_ns(ns))) > > + owning_ns = to_net_ns(ns)->user_ns; > > + break; > > +#endif > > +#ifdef CONFIG_PID_NS > > + case CLONE_NEWPID: > > + if (!current_in_namespace(to_pid_ns(ns))) { > > + owning_ns = to_pid_ns(ns)->user_ns; > > + } else if (!READ_ONCE(to_pid_ns(ns)->child_reaper)) { > > + ns->ops->put(ns); > > + return ERR_PTR(-EPERM); > > + } > > + break; > > +#endif > > +#ifdef CONFIG_TIME_NS > > + case CLONE_NEWTIME: > > + if (!current_in_namespace(to_time_ns(ns))) > > + owning_ns = to_time_ns(ns)->user_ns; > > + break; > > +#endif > > +#ifdef CONFIG_USER_NS > > + case CLONE_NEWUSER: > > + if (!current_in_namespace(to_user_ns(ns))) > > + owning_ns = to_user_ns(ns); > > + break; > > +#endif > > +#ifdef CONFIG_UTS_NS > > + case CLONE_NEWUTS: > > + if (!current_in_namespace(to_uts_ns(ns))) > > + owning_ns = to_uts_ns(ns)->user_ns; > > + break; > > +#endif > > Frankly, switches like these are asking for more Generic usage ;) But ok > for now. > > > + default: > > + return ERR_PTR(-EOPNOTSUPP); > > + } > > + > > + if (owning_ns && !ns_capable(owning_ns, CAP_SYS_ADMIN)) { > > + ns->ops->put(ns); > > + return ERR_PTR(-EPERM); > > + } > > + > > + /* path_from_stashed() unconditionally consumes the reference. */ > > + ret = path_from_stashed(&ns->stashed, nsfs_mnt, ns, &path); > > + if (ret) > > + return ERR_PTR(ret); > > + > > + return no_free_ptr(path.dentry); > > Ugh, so IMO this is very subtle because we declare > > struct path path __free(path_put) > > but then do no_free_ptr(path.dentry). I really had to lookup implementation > of no_free_ptr() to check whether we are leaking mnt reference here or not > (we are not). But that seems as an implementation detail we shouldn't > better rely on? Wouldn't be: > > return dget(path.dentry); > > much clearer (and sligthly less efficient, I know, but who cares)?
Fine by me as well!
