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authorAdrian Reber <areber@redhat.com>2019-11-15 13:36:20 +0100
committerChristian Brauner <christian.brauner@ubuntu.com>2019-11-15 23:49:22 +0100
commit49cb2fc42ce4b7a656ee605e30c302efaa39c1a7 (patch)
tree74f53f8f9ccc4b49aa06416ca5e28c0ffb9097f5 /kernel/pid.c
parent17a810699c189cb8f2f0ba21c7f83396599bea26 (diff)
fork: extend clone3() to support setting a PID
The main motivation to add set_tid to clone3() is CRIU. To restore a process with the same PID/TID CRIU currently uses /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to ns_last_pid and then (quickly) does a clone(). This works most of the time, but it is racy. It is also slow as it requires multiple syscalls. Extending clone3() to support *set_tid makes it possible restore a process using CRIU without accessing /proc/sys/kernel/ns_last_pid and race free (as long as the desired PID/TID is available). This clone3() extension places the same restrictions (CAP_SYS_ADMIN) on clone3() with *set_tid as they are currently in place for ns_last_pid. The original version of this change was using a single value for set_tid. At the 2019 LPC, after presenting set_tid, it was, however, decided to change set_tid to an array to enable setting the PID of a process in multiple PID namespaces at the same time. If a process is created in a PID namespace it is possible to influence the PID inside and outside of the PID namespace. Details also in the corresponding selftest. To create a process with the following PIDs: PID NS level Requested PID 0 (host) 31496 1 42 2 1 For that example the two newly introduced parameters to struct clone_args (set_tid and set_tid_size) would need to be: set_tid[0] = 1; set_tid[1] = 42; set_tid[2] = 31496; set_tid_size = 3; If only the PIDs of the two innermost nested PID namespaces should be defined it would look like this: set_tid[0] = 1; set_tid[1] = 42; set_tid_size = 2; The PID of the newly created process would then be the next available free PID in the PID namespace level 0 (host) and 42 in the PID namespace at level 1 and the PID of the process in the innermost PID namespace would be 1. The set_tid array is used to specify the PID of a process starting from the innermost nested PID namespaces up to set_tid_size PID namespaces. set_tid_size cannot be larger then the current PID namespace level. Signed-off-by: Adrian Reber <areber@redhat.com> Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com> Acked-by: Andrei Vagin <avagin@gmail.com> Link: https://lore.kernel.org/r/20191115123621.142252-1-areber@redhat.com Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Diffstat (limited to 'kernel/pid.c')
-rw-r--r--kernel/pid.c72
1 files changed, 57 insertions, 15 deletions
diff --git a/kernel/pid.c b/kernel/pid.c
index 7b5f6c963d72..2278e249141d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -157,7 +157,8 @@ void free_pid(struct pid *pid)
call_rcu(&pid->rcu, delayed_put_pid);
}
-struct pid *alloc_pid(struct pid_namespace *ns)
+struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
+ size_t set_tid_size)
{
struct pid *pid;
enum pid_type type;
@@ -166,6 +167,17 @@ struct pid *alloc_pid(struct pid_namespace *ns)
struct upid *upid;
int retval = -ENOMEM;
+ /*
+ * set_tid_size contains the size of the set_tid array. Starting at
+ * the most nested currently active PID namespace it tells alloc_pid()
+ * which PID to set for a process in that most nested PID namespace
+ * up to set_tid_size PID namespaces. It does not have to set the PID
+ * for a process in all nested PID namespaces but set_tid_size must
+ * never be greater than the current ns->level + 1.
+ */
+ if (set_tid_size > ns->level + 1)
+ return ERR_PTR(-EINVAL);
+
pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
if (!pid)
return ERR_PTR(retval);
@@ -174,24 +186,54 @@ struct pid *alloc_pid(struct pid_namespace *ns)
pid->level = ns->level;
for (i = ns->level; i >= 0; i--) {
- int pid_min = 1;
+ int tid = 0;
+
+ if (set_tid_size) {
+ tid = set_tid[ns->level - i];
+
+ retval = -EINVAL;
+ if (tid < 1 || tid >= pid_max)
+ goto out_free;
+ /*
+ * Also fail if a PID != 1 is requested and
+ * no PID 1 exists.
+ */
+ if (tid != 1 && !tmp->child_reaper)
+ goto out_free;
+ retval = -EPERM;
+ if (!ns_capable(tmp->user_ns, CAP_SYS_ADMIN))
+ goto out_free;
+ set_tid_size--;
+ }
idr_preload(GFP_KERNEL);
spin_lock_irq(&pidmap_lock);
- /*
- * init really needs pid 1, but after reaching the maximum
- * wrap back to RESERVED_PIDS
- */
- if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
- pid_min = RESERVED_PIDS;
-
- /*
- * Store a null pointer so find_pid_ns does not find
- * a partially initialized PID (see below).
- */
- nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
- pid_max, GFP_ATOMIC);
+ if (tid) {
+ nr = idr_alloc(&tmp->idr, NULL, tid,
+ tid + 1, GFP_ATOMIC);
+ /*
+ * If ENOSPC is returned it means that the PID is
+ * alreay in use. Return EEXIST in that case.
+ */
+ if (nr == -ENOSPC)
+ nr = -EEXIST;
+ } else {
+ int pid_min = 1;
+ /*
+ * init really needs pid 1, but after reaching the
+ * maximum wrap back to RESERVED_PIDS
+ */
+ if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
+ pid_min = RESERVED_PIDS;
+
+ /*
+ * Store a null pointer so find_pid_ns does not find
+ * a partially initialized PID (see below).
+ */
+ nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
+ pid_max, GFP_ATOMIC);
+ }
spin_unlock_irq(&pidmap_lock);
idr_preload_end();