Re: [PATCH 1/2] fs/proc: do_task_stat: Fix ESP not readable during coredump

[Oleg Nesterov <oleg@redhat.com>]

On 11/06, Nam Cao wrote:
>
> @@ -534,6 +517,23 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>  		ppid = task_tgid_nr_ns(task->real_parent, ns);
>  		pgid = task_pgrp_nr_ns(task, ns);
>
> +		/*
> +		 * esp and eip are intentionally zeroed out.  There is no
> +		 * non-racy way to read them without freezing the task.
> +		 * Programs that need reliable values can use ptrace(2).

OK,

but then:

> +		 * The only exception is if the task is core dumping because
> +		 * a program is not able to use ptrace(2) in that case. It is
> +		 * safe because the task has stopped executing permanently.
> +		 */
> +		if (permitted && task->signal->core_state) {
> +			if (try_get_task_stack(task)) {
> +				eip = KSTK_EIP(task);
> +				esp = KSTK_ESP(task);
> +				put_task_stack(task);

How can the task->signal->core_state check help ?

Suppose we have a task T1 with T1-pid == 100 and you read /proc/100/stat.
It is possible that the T1's sub-thread T2 starts the coredumping and sets
signal->core_state != NULL.

But read(/proc/100/stat) can run before T1 gets SIGKILL from T2 and enters
the kernel mode?

Oleg.