Re: [PATCH 2/2] ftrace: fix race in function graph during fork

[Frédéric Weisbecker]

2008/12/3 Steven Rostedt <rostedt@xxxxxxxxxxx>:
> From: Steven Rostedt <srostedt@xxxxxxxxxx>
>
> Impact: fix
>
> There is a nasy race in startup of a new process running the
> function graph tracer. In fork.c:
>
>        total_forks++;
>        spin_unlock(&current->sighand->siglock);
>        write_unlock_irq(&tasklist_lock);
>        ftrace_graph_init_task(p);
>        proc_fork_connector(p);
>        cgroup_post_fork(p);
>        return p;
>
> The new task is free to run as soon as the tasklist_lock is released.
> This is before the ftrace_graph_init_task. If the task does run
> it will be using the same ret_stack and curr_ret_stack as the parent.
> This will cause horrible crashes that are nasty to debug.
>
> This patch moves the ftrace_graph_init_task to just after the alloc_pid
> code. This fixes the above race.
>
> Signed-off-by: Steven Rostedt <srostedt@xxxxxxxxxx>
> ---
>  kernel/fork.c |    9 ++++++---
>  1 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index afb376d..91cca76 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1142,6 +1142,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>                }
>        }
>
> +       ftrace_graph_init_task(p);
> +
>        p->pid = pid_nr(pid);
>        p->tgid = p->pid;
>        if (clone_flags & CLONE_THREAD)
> @@ -1150,7 +1152,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>        if (current->nsproxy != p->nsproxy) {
>                retval = ns_cgroup_clone(p, pid);
>                if (retval)
> -                       goto bad_fork_free_pid;
> +                       goto bad_fork_free_graph;
>        }
>
>        p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
> @@ -1243,7 +1245,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>                spin_unlock(&current->sighand->siglock);
>                write_unlock_irq(&tasklist_lock);
>                retval = -ERESTARTNOINTR;
> -               goto bad_fork_free_pid;
> +               goto bad_fork_free_graph;
>        }
>
>        if (clone_flags & CLONE_THREAD) {
> @@ -1276,11 +1278,12 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>        total_forks++;
>        spin_unlock(&current->sighand->siglock);
>        write_unlock_irq(&tasklist_lock);
> -       ftrace_graph_init_task(p);
>        proc_fork_connector(p);
>        cgroup_post_fork(p);
>        return p;
>
> +bad_fork_free_graph:
> +       ftrace_graph_exit_task(p);
>  bad_fork_free_pid:
>        if (pid != &init_struct_pid)
>                free_pid(pid);


Very nice catch! Thanks.
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