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    SubjectRe: [PATCH] nsfs: add NS_GET_INIT_PID ioctl
    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@proxmox.com>
    > > Cc: Serge Hallyn <serge@hallyn.com>
    > > Cc: Michael Kerrisk <mtk.manpages@gmail.com>
    > > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    > > Cc: linux-fsdevel@vger.kernel.org
    > > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
    >
    > 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

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