exit: reparent: use ->ptrace_entry rather than ->sibling for EXIT_DEAD tasks

oleg-nesterov · torvalds · commit dc2fd4b00946 · 2014-12-10T17:41:10.000-08:00
reparent_leader() reuses -&gt;sibling as a list node to add an EXIT_DEAD task
into dead_children list we are going to release.  This obviously removes
the dead task from its real_parent-&gt;children list and this is even good;
the parent can do nothing with the EXIT_DEAD reparented zombie, it only
makes do_wait() slower.

But, this also means that it can not be reparented once again, so if its
new parent dies too nobody will update -&gt;parent/real_parent, they can
point to the freed memory even before release_task() we are going to call,
this breaks the code which relies on pid_alive() to access
-&gt;real_parent/parent.

Fortunately this is mostly theoretical, this can only happen if init or
PR_SET_CHILD_SUBREAPER process ignores SIGCHLD and the new parent
sub-thread exits right after we drop tasklist_lock.

Change this code to use -&gt;ptrace_entry instead, we know that the child is
not traced so nobody can ever use this member.  This also allows to unify
this logic with exit_ptrace(), see the next changes.

Note: we really need to change release_task() to nullify real_parent/
parent/group_leader pointers, but we need to change the current users
first somehow.  And it would be better to reap this zombie immediately but
release_task_locked() we need is complicated by proc_flush_task().

Signed-off-by: Oleg Nesterov &lt;oleg@redhat.com&gt;
Cc: Aaron Tomlin &lt;atomlin@redhat.com&gt;
Cc: Alexey Dobriyan &lt;adobriyan@gmail.com&gt;
Cc: "Eric W. Biederman" &lt;ebiederm@xmission.com&gt;,
Cc: Sterling Alexander &lt;stalexan@redhat.com&gt;
Cc: Peter Zijlstra &lt;peterz@infradead.org&gt;
Cc: Roland McGrath &lt;roland@hack.frob.com&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
diff --git a/kernel/exit.c b/kernel/exit.c
@@ -548,7 +548,7 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
 	    p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
 		if (do_notify_parent(p, p->exit_signal)) {
 			p->exit_state = EXIT_DEAD;
-			list_move_tail(&p->sibling, dead);
+			list_add(&p->ptrace_entry, dead);
 		}
 	}
 
@@ -587,8 +587,8 @@ static void forget_original_parent(struct task_struct *father)
 
 	BUG_ON(!list_empty(&father->children));
 
-	list_for_each_entry_safe(p, n, &dead_children, sibling) {
-		list_del_init(&p->sibling);
+	list_for_each_entry_safe(p, n, &dead_children, ptrace_entry) {
+		list_del_init(&p->ptrace_entry);
 		release_task(p);
 	}
 }

Original file line number	Diff line number	Diff line change
`@@ -548,7 +548,7 @@ static void reparent_leader(struct task_struct father, struct task_struct p,`
`548`	`548`	`p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {`
`549`	`549`	`if (do_notify_parent(p, p->exit_signal)) {`
`550`	`550`	`p->exit_state = EXIT_DEAD;`
`551`		`- list_move_tail(&p->sibling, dead);`
	`551`	`+ list_add(&p->ptrace_entry, dead);`
`552`	`552`	`}`
`553`	`553`	`}`
`554`	`554`
`@@ -587,8 +587,8 @@ static void forget_original_parent(struct task_struct *father)`
`587`	`587`
`588`	`588`	`BUG_ON(!list_empty(&father->children));`
`589`	`589`
`590`		`- list_for_each_entry_safe(p, n, &dead_children, sibling) {`
`591`		`- list_del_init(&p->sibling);`
	`590`	`+ list_for_each_entry_safe(p, n, &dead_children, ptrace_entry) {`
	`591`	`+ list_del_init(&p->ptrace_entry);`
`592`	`592`	`release_task(p);`
`593`	`593`	`}`
`594`	`594`	`}`