On Fri, Jul 10, 2020 at 07:58:36AM -0400, Qian Cai wrote: > On Thu, Jun 18, 2020 at 10:45:43AM +0200, Christian Brauner wrote: > > Add an ioctl() to return the PID of the init process/child reaper of a pid > > namespace as seen in the caller's pid namespace. > > > > LXCFS is a tiny fuse filesystem used to virtualize various aspects of > > procfs. It is used actively by a large number of users including ChromeOS > > and cloud providers. LXCFS is run on the host. The files and directories it > > creates can be bind-mounted by e.g. a container at startup and mounted over > > the various procfs files the container wishes to have virtualized. When > > e.g. a read request for uptime is received, LXCFS will receive the pid of > > the reader. In order to virtualize the corresponding read, LXCFS needs to > > know the pid of the init process of the reader's pid namespace. In order to > > do this, LXCFS first needs to fork() two helper processes. The first helper > > process setns() to the readers pid namespace. The second helper process is > > needed to create a process that is a proper member of the pid namespace. > > The second helper process then creates a ucred message with ucred.pid set > > to 1 and sends it back to LXCFS. The kernel will translate the ucred.pid > > field to the corresponding pid number in LXCFS's pid namespace. This way > > LXCFS can learn the init pid number of the reader's pid namespace and can > > go on to virtualize. Since these two forks() are costly LXCFS maintains an > > init pid cache that caches a given pid for a fixed amount of time. The > > cache is pruned during new read requests. However, even with the cache the > > hit of the two forks() is singificant when a very large number of > > containers are running. With this simple patch we add an ns ioctl that > > let's a caller retrieve the init pid nr of a pid namespace through its > > pid namespace fd. This _significantly_ improves our performance with a very > > simple change. A caller should do something like: > > - pid_t init_pid = ioctl(pid_ns_fd, NS_GET_INIT_PID); > > - verify init_pid is still valid (not necessarily both but recommended): > > - opening a pidfd to get a stable reference > > - opening /proc/<init_pid>/ns/pid and verifying that <pid_ns_fd> > > and the pid namespace fd of <init_pid> refer to the same pid namespace > > > > Note, it is possible for the init process of the pid namespace (identified > > via the child_reaper member in the relevant pid namespace) to die and get > > reaped right after the ioctl returned. If that happens there are two cases > > to consider: > > - if the init process was single threaded, all other processes in the pid > > namespace will be zapped and any new process creation in there will fail; > > A caller can detect this case since either the init pid is still around > > but it is a zombie, or it already has exited and not been recycled, or it > > has exited, been reaped, and also been recycled. The last case is the > > most interesting one but a caller would then be able to detect that the > > recycled process lives in a different pid namespace. > > - if the init process was multi-threaded, then the kernel will try to make > > one of the threads in the same thread-group - if any are still alive - > > the new child_reaper. In this case the caller can detect that the thread > > which exited and used to be the child_reaper is no longer alive. If it's > > tid has been recycled in the same pid namespace a caller can detect this > > by parsing through /proc/<tid>/stat, looking at the Nspid: field and if > > there's a entry with pid nr 1 in the respective pid namespace it can be > > sure that it hasn't been recycled. > > Both options can be combined with pidfd_open() to make sure that a stable > > reference is maintained. > > > > Cc: Wolfgang Bumiller <w.bumiller@xxxxxxxxxxx> > > Cc: Serge Hallyn <serge@xxxxxxxxxx> > > Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx> > > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> > > Cc: linux-fsdevel@xxxxxxxxxxxxxxx > > Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx> > > fs/nsfs.c: In function ‘ns_ioctl’: > fs/nsfs.c:195:14: warning: unused variable ‘pid_struct’ [-Wunused-variable] > struct pid *pid_struct; > ^~~~~~~~~~ > fs/nsfs.c:194:22: warning: unused variable ‘child_reaper’ [-Wunused-variable] > struct task_struct *child_reaper; > ^~~~~~~~~~~~ Thanks. Fyi, this has been reported by Stephen already. Christian
