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, 16 insertions, 1 deletions

diff --git a/kernel/utsname.c b/kernel/utsname.c
index fdc619eb61ef..f6336d51d64c 100644
--- a/kernel/utsname.c
+++ b/kernel/utsname.c
@@ -36,11 +36,18 @@ static struct uts_namespace *clone_uts_ns(struct user_namespace *user_ns,
                                           struct uts_namespace *old_ns)
 {
         struct uts_namespace *ns;
+        int err;
 
         ns = create_uts_ns();
         if (!ns)
                 return ERR_PTR(-ENOMEM);
 
+        err = proc_alloc_inum(&ns->proc_inum);
+        if (err) {
+                kfree(ns);
+                return ERR_PTR(err);
+        }
+
         down_read(&uts_sem);
         memcpy(&ns->name, &old_ns->name, sizeof(ns->name));
         ns->user_ns = get_user_ns(user_ns);
@@ -77,6 +84,7 @@ void free_uts_ns(struct kref *kref)
 
         ns = container_of(kref, struct uts_namespace, kref);
         put_user_ns(ns->user_ns);
+        proc_free_inum(ns->proc_inum);
         kfree(ns);
 }
 
@@ -114,11 +122,18 @@ static int utsns_install(struct nsproxy *nsproxy, void *new)
         return 0;
 }
 
+static unsigned int utsns_inum(void *vp)
+{
+        struct uts_namespace *ns = vp;
+
+        return ns->proc_inum;
+}
+
 const struct proc_ns_operations utsns_operations = {
         .name           = "uts",
         .type           = CLONE_NEWUTS,
         .get            = utsns_get,
         .put            = utsns_put,
         .install        = utsns_install,
+        .inum           = utsns_inum,
 };
-