To avoid potential problems related to cpu/compiler optimizations around ->child_reaper, let's use WRITE_ONCE (additional to task_list lock) everywhere we write it and use READ_ONCE where we read it without explicit lock. Note: It also pairs with existing READ_ONCE with no lock in nsfs_fh_to_dentry().
Also let's add ASSERT_EXCLUSIVE_WRITER before write to identify to KCSAN that we don't expect any concurrent ->child_reaper modifications, and those must be detected. Suggested-by: Oleg Nesterov <[email protected]> Acked-by: Oleg Nesterov <[email protected]> Signed-off-by: Pavel Tikhomirov <[email protected]> -- v3: Split from main commit. Add ASSERT_EXCLUSIVE_WRITER. v5: Add one more READ_ONCE for access without lock in free_pid(). --- kernel/exit.c | 3 ++- kernel/fork.c | 5 ++++- kernel/pid.c | 4 ++-- 3 files changed, 8 insertions(+), 4 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 1f32023d0dbe..25e9cb6de7e7 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -608,7 +608,8 @@ static struct task_struct *find_child_reaper(struct task_struct *father, reaper = find_alive_thread(father); if (reaper) { - pid_ns->child_reaper = reaper; + ASSERT_EXCLUSIVE_WRITER(pid_ns->child_reaper); + WRITE_ONCE(pid_ns->child_reaper, reaper); return reaper; } diff --git a/kernel/fork.c b/kernel/fork.c index 7d369a8a2ad0..19a75a041e6a 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2428,7 +2428,10 @@ __latent_entropy struct task_struct *copy_process( init_task_pid(p, PIDTYPE_SID, task_session(current)); if (is_child_reaper(pid)) { - ns_of_pid(pid)->child_reaper = p; + struct pid_namespace *ns = ns_of_pid(pid); + + ASSERT_EXCLUSIVE_WRITER(ns->child_reaper); + WRITE_ONCE(ns->child_reaper, p); p->signal->flags |= SIGNAL_UNKILLABLE; } p->signal->shared_pending.signal = delayed.signal; diff --git a/kernel/pid.c b/kernel/pid.c index 2f1dbcbc2349..bb2bb6c16498 100644 --- a/kernel/pid.c +++ b/kernel/pid.c @@ -128,7 +128,7 @@ void free_pid(struct pid *pid) * is the reaper wake up the reaper. The reaper * may be sleeping in zap_pid_ns_processes(). */ - wake_up_process(ns->child_reaper); + wake_up_process(READ_ONCE(ns->child_reaper)); break; case PIDNS_ADDING: /* Only possible if the 1st fork fails */ @@ -218,7 +218,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid, * Also fail if a PID != 1 is requested and * no PID 1 exists. */ - if (tid != 1 && !tmp->child_reaper) + if (tid != 1 && !READ_ONCE(tmp->child_reaper)) goto out_abort; retval = -EPERM; if (!checkpoint_restore_ns_capable(tmp->user_ns)) -- 2.53.0
