proc: Usable inode numbers for the namespace file descriptors.

Assign a unique proc inode to each namespace, and use that
inode number to ensure we only allocate at most one proc
inode for every namespace in proc.

A single proc inode per namespace allows userspace to test
to see if two processes are in the same namespace.

This has been a long requested feature and only blocked because
a naive implementation would put the id in a global space and
would ultimately require having a namespace for the names of
namespaces, making migration and certain virtualization tricks
impossible.

We still don't have per superblock inode numbers for proc, which
appears necessary for application unaware checkpoint/restart and
migrations (if the application is using namespace file descriptors)
but that is now allowd by the design if it becomes important.

I have preallocated the ipc and uts initial proc inode numbers so
their structures can be statically initialized.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index cab78a74aca3..c1bbe86f4920 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2301,6 +2301,7 @@ dput_out:
 
 static void free_mnt_ns(struct mnt_namespace *ns)
 {
+        proc_free_inum(ns->proc_inum);
         put_user_ns(ns->user_ns);
         kfree(ns);
 }
@@ -2317,10 +2318,16 @@ static atomic64_t mnt_ns_seq = ATOMIC64_INIT(1);
 static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
 {
         struct mnt_namespace *new_ns;
+        int ret;
 
         new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
         if (!new_ns)
                 return ERR_PTR(-ENOMEM);
+        ret = proc_alloc_inum(&new_ns->proc_inum);
+        if (ret) {
+                kfree(new_ns);
+                return ERR_PTR(ret);
+        }
         new_ns->seq = atomic64_add_return(1, &mnt_ns_seq);
         atomic_set(&new_ns->count, 1);
         new_ns->root = NULL;
@@ -2799,10 +2806,17 @@ static int mntns_install(struct nsproxy *nsproxy, void *ns)
         return 0;
 }
 
+static unsigned int mntns_inum(void *ns)
+{
+        struct mnt_namespace *mnt_ns = ns;
+        return mnt_ns->proc_inum;
+}
+
 const struct proc_ns_operations mntns_operations = {
         .name           = "mnt",
         .type           = CLONE_NEWNS,
         .get            = mntns_get,
         .put            = mntns_put,
         .install        = mntns_install,
+        .inum           = mntns_inum,
 };