ptrace children revamp

ptrace no longer fiddles with the children/sibling links, and the
old ptrace_children list is gone.  Now ptrace, whether of one's own
children or another's via PTRACE_ATTACH, just uses the new ptraced
list instead.

There should be no user-visible difference that matters.  The only
change is the order in which do_wait() sees multiple stopped
children and stopped ptrace attachees.  Since wait_task_stopped()
was changed earlier so it no longer reorders the children list, we
already know this won't cause any new problems.

Signed-off-by: Roland McGrath <roland@redhat.com>

3 files changed, 146 insertions, 123 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 7453356a961f..1e909826a804 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -71,7 +71,7 @@ static void __unhash_process(struct task_struct *p)
                 __get_cpu_var(process_counts)--;
         }
         list_del_rcu(&p->thread_group);
-        remove_parent(p);
+        list_del_init(&p->sibling);
 }
 
 /*
@@ -152,6 +152,18 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
         put_task_struct(container_of(rhp, struct task_struct, rcu));
 }
 
+/*
+ * Do final ptrace-related cleanup of a zombie being reaped.
+ *
+ * Called with write_lock(&tasklist_lock) held.
+ */
+static void ptrace_release_task(struct task_struct *p)
+{
+        BUG_ON(!list_empty(&p->ptraced));
+        ptrace_unlink(p);
+        BUG_ON(!list_empty(&p->ptrace_entry));
+}
+
 void release_task(struct task_struct * p)
 {
         struct task_struct *leader;
@@ -160,8 +172,7 @@ repeat:
         atomic_dec(&p->user->processes);
         proc_flush_task(p);
         write_lock_irq(&tasklist_lock);
-        ptrace_unlink(p);
-        BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
+        ptrace_release_task(p);
         __exit_signal(p);
 
         /*
@@ -315,9 +326,8 @@ static void reparent_to_kthreadd(void)
 
         ptrace_unlink(current);
         /* Reparent to init */
-        remove_parent(current);
         current->real_parent = current->parent = kthreadd_task;
-        add_parent(current);
+        list_move_tail(&current->sibling, &current->real_parent->children);
 
         /* Set the exit signal to SIGCHLD so we signal init on exit */
         current->exit_signal = SIGCHLD;
@@ -692,37 +702,71 @@ static void exit_mm(struct task_struct * tsk)
         mmput(mm);
 }
 
-static void
-reparent_thread(struct task_struct *p, struct task_struct *father, int traced)
+/*
+ * Detach all tasks we were using ptrace on.
+ * Any that need to be release_task'd are put on the @dead list.
+ *
+ * Called with write_lock(&tasklist_lock) held.
+ */
+static void ptrace_exit(struct task_struct *parent, struct list_head *dead)
 {
-        if (p->pdeath_signal)
-                /* We already hold the tasklist_lock here.  */
-                group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
+        struct task_struct *p, *n;
 
-        /* Move the child from its dying parent to the new one.  */
-        if (unlikely(traced)) {
-                /* Preserve ptrace links if someone else is tracing this child.  */
-                list_del_init(&p->ptrace_list);
-                if (ptrace_reparented(p))
-                        list_add(&p->ptrace_list, &p->real_parent->ptrace_children);
-        } else {
-                /* If this child is being traced, then we're the one tracing it
-                 * anyway, so let go of it.
+        list_for_each_entry_safe(p, n, &parent->ptraced, ptrace_entry) {
+                __ptrace_unlink(p);
+
+                if (p->exit_state != EXIT_ZOMBIE)
+                        continue;
+
+                /*
+                 * If it's a zombie, our attachedness prevented normal
+                 * parent notification or self-reaping.  Do notification
+                 * now if it would have happened earlier.  If it should
+                 * reap itself, add it to the @dead list.  We can't call
+                 * release_task() here because we already hold tasklist_lock.
+                 *
+                 * If it's our own child, there is no notification to do.
                  */
-                p->ptrace = 0;
-                remove_parent(p);
-                p->parent = p->real_parent;
-                add_parent(p);
+                if (!task_detached(p) && thread_group_empty(p)) {
+                        if (!same_thread_group(p->real_parent, parent))
+                                do_notify_parent(p, p->exit_signal);
+                }
 
-                if (task_is_traced(p)) {
+                if (task_detached(p)) {
                         /*
-                         * If it was at a trace stop, turn it into
-                         * a normal stop since it's no longer being
-                         * traced.
+                         * Mark it as in the process of being reaped.
                          */
-                        ptrace_untrace(p);
+                        p->exit_state = EXIT_DEAD;
+                        list_add(&p->ptrace_entry, dead);
                 }
         }
+}
+
+/*
+ * Finish up exit-time ptrace cleanup.
+ *
+ * Called without locks.
+ */
+static void ptrace_exit_finish(struct task_struct *parent,
+                               struct list_head *dead)
+{
+        struct task_struct *p, *n;
+
+        BUG_ON(!list_empty(&parent->ptraced));
+
+        list_for_each_entry_safe(p, n, dead, ptrace_entry) {
+                list_del_init(&p->ptrace_entry);
+                release_task(p);
+        }
+}
+
+static void reparent_thread(struct task_struct *p, struct task_struct *father)
+{
+        if (p->pdeath_signal)
+                /* We already hold the tasklist_lock here.  */
+                group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
+
+        list_move_tail(&p->sibling, &p->real_parent->children);
 
         /* If this is a threaded reparent there is no need to
          * notify anyone anything has happened.
@@ -737,7 +781,8 @@ reparent_thread(struct task_struct *p, struct task_struct *father, int traced)
         /* If we'd notified the old parent about this child's death,
          * also notify the new parent.
          */
-        if (!traced && p->exit_state == EXIT_ZOMBIE &&
+        if (!ptrace_reparented(p) &&
+            p->exit_state == EXIT_ZOMBIE &&
             !task_detached(p) && thread_group_empty(p))
                 do_notify_parent(p, p->exit_signal);
 
@@ -754,12 +799,15 @@ reparent_thread(struct task_struct *p, struct task_struct *father, int traced)
 static void forget_original_parent(struct task_struct *father)
 {
         struct task_struct *p, *n, *reaper = father;
-        struct list_head ptrace_dead;
-
-        INIT_LIST_HEAD(&ptrace_dead);
+        LIST_HEAD(ptrace_dead);
 
         write_lock_irq(&tasklist_lock);
 
+        /*
+         * First clean up ptrace if we were using it.
+         */
+        ptrace_exit(father, &ptrace_dead);
+
         do {
                 reaper = next_thread(reaper);
                 if (reaper == father) {
@@ -768,58 +816,19 @@ static void forget_original_parent(struct task_struct *father)
                 }
         } while (reaper->flags & PF_EXITING);
 
-        /*
-         * There are only two places where our children can be:
-         *
-         * - in our child list
-         * - in our ptraced child list
-         *
-         * Search them and reparent children.
-         */
         list_for_each_entry_safe(p, n, &father->children, sibling) {
-                int ptrace;
-
-                ptrace = p->ptrace;
-
-                /* if father isn't the real parent, then ptrace must be enabled */
-                BUG_ON(father != p->real_parent && !ptrace);
-
-                if (father == p->real_parent) {
-                        /* reparent with a reaper, real father it's us */
-                        p->real_parent = reaper;
-                        reparent_thread(p, father, 0);
-                } else {
-                        /* reparent ptraced task to its real parent */
-                        __ptrace_unlink (p);
-                        if (p->exit_state == EXIT_ZOMBIE && !task_detached(p) &&
-                            thread_group_empty(p))
-                                do_notify_parent(p, p->exit_signal);
-                }
-
-                /*
-                 * if the ptraced child is a detached zombie we must collect
-                 * it before we exit, or it will remain zombie forever since
-                 * we prevented it from self-reap itself while it was being
-                 * traced by us, to be able to see it in wait4.
-                 */
-                if (unlikely(ptrace && p->exit_state == EXIT_ZOMBIE && task_detached(p)))
-                        list_add(&p->ptrace_list, &ptrace_dead);
-        }
-
-        list_for_each_entry_safe(p, n, &father->ptrace_children, ptrace_list) {
                 p->real_parent = reaper;
-                reparent_thread(p, father, 1);
+                if (p->parent == father) {
+                        BUG_ON(p->ptrace);
+                        p->parent = p->real_parent;
+                }
+                reparent_thread(p, father);
         }
 
         write_unlock_irq(&tasklist_lock);
         BUG_ON(!list_empty(&father->children));
-        BUG_ON(!list_empty(&father->ptrace_children));
-
-        list_for_each_entry_safe(p, n, &ptrace_dead, ptrace_list) {
-                list_del_init(&p->ptrace_list);
-                release_task(p);
-        }
 
+        ptrace_exit_finish(father, &ptrace_dead);
 }
 
 /*
@@ -1180,13 +1189,6 @@ static int eligible_child(enum pid_type type, struct pid *pid, int options,
                         return 0;
         }
 
-        /*
-         * Do not consider detached threads that are
-         * not ptraced:
-         */
-        if (task_detached(p) && !p->ptrace)
-                return 0;
-
         /* Wait for all children (clone and not) if __WALL is set;
          * otherwise, wait for clone children *only* if __WCLONE is
          * set; otherwise, wait for non-clone children *only*.  (Note:
@@ -1399,7 +1401,7 @@ static int wait_task_zombie(struct task_struct *p, int options,
  * the lock and this task is uninteresting.  If we return nonzero, we have
  * released the lock and the system call should return.
  */
-static int wait_task_stopped(struct task_struct *p,
+static int wait_task_stopped(int ptrace, struct task_struct *p,
                              int options, struct siginfo __user *infop,
                              int __user *stat_addr, struct rusage __user *ru)
 {
@@ -1407,7 +1409,7 @@ static int wait_task_stopped(struct task_struct *p,
         uid_t uid = 0; /* unneeded, required by compiler */
         pid_t pid;
 
-        if (!(p->ptrace & PT_PTRACED) && !(options & WUNTRACED))
+        if (!(options & WUNTRACED))
                 return 0;
 
         exit_code = 0;
@@ -1416,7 +1418,7 @@ static int wait_task_stopped(struct task_struct *p,
         if (unlikely(!task_is_stopped_or_traced(p)))
                 goto unlock_sig;
 
-        if (!(p->ptrace & PT_PTRACED) && p->signal->group_stop_count > 0)
+        if (!ptrace && p->signal->group_stop_count > 0)
                 /*
                  * A group stop is in progress and this is the group leader.
                  * We won't report until all threads have stopped.
@@ -1445,7 +1447,7 @@ unlock_sig:
          */
         get_task_struct(p);
         pid = task_pid_vnr(p);
-        why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
+        why = ptrace ? CLD_TRAPPED : CLD_STOPPED;
         read_unlock(&tasklist_lock);
 
         if (unlikely(options & WNOWAIT))
@@ -1536,7 +1538,7 @@ static int wait_task_continued(struct task_struct *p, int options,
  * Returns zero if the search for a child should continue;
  * then *@notask_error is 0 if @p is an eligible child, or still -ECHILD.
  */
-static int wait_consider_task(struct task_struct *parent,
+static int wait_consider_task(struct task_struct *parent, int ptrace,
                               struct task_struct *p, int *notask_error,
                               enum pid_type type, struct pid *pid, int options,
                               struct siginfo __user *infop,
@@ -1546,6 +1548,15 @@ static int wait_consider_task(struct task_struct *parent,
         if (ret <= 0)
                 return ret;
 
+        if (likely(!ptrace) && unlikely(p->ptrace)) {
+                /*
+                 * This child is hidden by ptrace.
+                 * We aren't allowed to see it now, but eventually we will.
+                 */
+                *notask_error = 0;
+                return 0;
+        }
+
         if (p->exit_state == EXIT_DEAD)
                 return 0;
 
@@ -1562,7 +1573,8 @@ static int wait_consider_task(struct task_struct *parent,
         *notask_error = 0;
 
         if (task_is_stopped_or_traced(p))
-                return wait_task_stopped(p, options, infop, stat_addr, ru);
+                return wait_task_stopped(ptrace, p, options,
+                                         infop, stat_addr, ru);
 
         return wait_task_continued(p, options, infop, stat_addr, ru);
 }
@@ -1583,11 +1595,16 @@ static int do_wait_thread(struct task_struct *tsk, int *notask_error,
         struct task_struct *p;
 
         list_for_each_entry(p, &tsk->children, sibling) {
-                int ret = wait_consider_task(tsk, p, notask_error,
-                                             type, pid, options,
-                                             infop, stat_addr, ru);
-                if (ret)
-                        return ret;
+                /*
+                 * Do not consider detached threads.
+                 */
+                if (!task_detached(p)) {
+                        int ret = wait_consider_task(tsk, 0, p, notask_error,
+                                                     type, pid, options,
+                                                     infop, stat_addr, ru);
+                        if (ret)
+                                return ret;
+                }
         }
 
         return 0;
@@ -1601,21 +1618,16 @@ static int ptrace_do_wait(struct task_struct *tsk, int *notask_error,
         struct task_struct *p;
 
         /*
-         * If we never saw an eligile child, check for children stolen by
-         * ptrace.  We don't leave -ECHILD in *@notask_error if there are any,
-         * because we will eventually be allowed to wait for them again.
+         * Traditionally we see ptrace'd stopped tasks regardless of options.
          */
-        if (!*notask_error)
-                return 0;
+        options |= WUNTRACED;
 
-        list_for_each_entry(p, &tsk->ptrace_children, ptrace_list) {
-                int ret = eligible_child(type, pid, options, p);
-                if (unlikely(ret < 0))
+        list_for_each_entry(p, &tsk->ptraced, ptrace_entry) {
+                int ret = wait_consider_task(tsk, 1, p, notask_error,
+                                             type, pid, options,
+                                             infop, stat_addr, ru);
+                if (ret)
                         return ret;
-                if (ret) {
-                        *notask_error = 0;
-                        return 0;
-                }
         }
 
         return 0;
diff --git a/kernel/fork.c b/kernel/fork.c
index 4bd2f516401f..adefc1131f27 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1125,8 +1125,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
          */
         p->group_leader = p;
         INIT_LIST_HEAD(&p->thread_group);
-        INIT_LIST_HEAD(&p->ptrace_children);
-        INIT_LIST_HEAD(&p->ptrace_list);
+        INIT_LIST_HEAD(&p->ptrace_entry);
+        INIT_LIST_HEAD(&p->ptraced);
 
         /* Now that the task is set up, run cgroup callbacks if
          * necessary. We need to run them before the task is visible
@@ -1198,7 +1198,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         }
 
         if (likely(p->pid)) {
-                add_parent(p);
+                list_add_tail(&p->sibling, &p->real_parent->children);
                 if (unlikely(p->ptrace & PT_PTRACED))
                         __ptrace_link(p, current->parent);
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index e337390fce01..8392a9da6450 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -33,13 +33,9 @@
  */
 void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
 {
-        BUG_ON(!list_empty(&child->ptrace_list));
-        if (child->parent == new_parent)
-                return;
-        list_add(&child->ptrace_list, &child->parent->ptrace_children);
-        remove_parent(child);
+        BUG_ON(!list_empty(&child->ptrace_entry));
+        list_add(&child->ptrace_entry, &new_parent->ptraced);
         child->parent = new_parent;
-        add_parent(child);
 }
  
 /*
@@ -73,12 +69,8 @@ void __ptrace_unlink(struct task_struct *child)
         BUG_ON(!child->ptrace);
 
         child->ptrace = 0;
-        if (ptrace_reparented(child)) {
-                list_del_init(&child->ptrace_list);
-                remove_parent(child);
-                child->parent = child->real_parent;
-                add_parent(child);
-        }
+        child->parent = child->real_parent;
+        list_del_init(&child->ptrace_entry);
 
         if (task_is_traced(child))
                 ptrace_untrace(child);
@@ -492,15 +484,34 @@ int ptrace_traceme(void)
         /*
          * Are we already being traced?
          */
+repeat:
         task_lock(current);
         if (!(current->ptrace & PT_PTRACED)) {
+                /*
+                 * See ptrace_attach() comments about the locking here.
+                 */
+                unsigned long flags;
+                if (!write_trylock_irqsave(&tasklist_lock, flags)) {
+                        task_unlock(current);
+                        do {
+                                cpu_relax();
+                        } while (!write_can_lock(&tasklist_lock));
+                        goto repeat;
+                }
+
                 ret = security_ptrace(current->parent, current,
                                       PTRACE_MODE_ATTACH);
+
                 /*
                  * Set the ptrace bit in the process ptrace flags.
+                 * Then link us on our parent's ptraced list.
                  */
-                if (!ret)
+                if (!ret) {
                         current->ptrace |= PT_PTRACED;
+                        __ptrace_link(current, current->real_parent);
+                }
+
+                write_unlock_irqrestore(&tasklist_lock, flags);
         }
         task_unlock(current);
         return ret;