Re: [RFC 5/5 v2] uprobes: add global breakpoints

[Oleg Nesterov <oleg@redhat.com>]

On 08/21, Sebastian Andrzej Siewior wrote:
>
> This patch adds the ability to hold the program once this point has been
> passed and the user may attach to the program via ptrace.

Sorry Sebastian, I didn't even try to read the patch ;) Fortunately I am
not maintainer, I can only reapeat that you do not need to convince me.

> Oleg: The change in ptrace_attach() is still as it was. I tried to
> address Peter concern here.
> Now what options do I have here:
> - not putting the task in TASK_TRACED but simply halt. This would work
>   without a change to ptrace_attach() but the task continues on any
>   signal. So a signal friendly task would continue and not notice a
>   thing.

TASK_KILLABLE

> - putting the TASK_TRACED

This is simply wrong, in many ways.

For example, what if the probed task is already ptraced? Or debugger
attaches via PTRACE_SEIZE? How can debugger know it is stopped?
uprobe_wait_traced() goes to sleep in TASK_TRACED without notification.
And it does not set ->exit_code, this means do_wait() won't work.
And note ptrace_stop()->recalc_sigpending_tsk().

> @@ -76,6 +79,7 @@ struct uprobe_task {
>
>  	unsigned long			xol_vaddr;
>  	unsigned long			vaddr;
> +	int				skip_handler;

I am trying to guess what this skip_handler does...

> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1513,7 +1513,16 @@ static void handle_swbp(struct pt_regs *regs)
>  			goto cleanup_ret;
>  	}
>  	utask->active_uprobe = uprobe;
> -	handler_chain(uprobe, regs);
> +	if (utask->skip_handler)
> +		utask->skip_handler = 0;
> +	else
> +		handler_chain(uprobe, regs);
> +
> +	if (utask->state == UTASK_TRACE_WOKEUP_TRACED) {
> +		send_sig(SIGTRAP, current, 0);
> +		utask->skip_handler = 1;
> +		goto cleanup_ret;
> +	}
>  	if (uprobe->flags & UPROBE_SKIP_SSTEP && can_skip_sstep(uprobe, regs))
>  		goto cleanup_ret;
>  
> @@ -1528,7 +1537,7 @@ cleanup_ret:
>  		utask->active_uprobe = NULL;
>  		utask->state = UTASK_RUNNING;
>  	}
> -	if (!(uprobe->flags & UPROBE_SKIP_SSTEP))
> +	if (!(uprobe->flags & UPROBE_SKIP_SSTEP) || utask->skip_handler)

Am I understand correctly?

If it was woken by PTRACE_ATTACH we set utask->skip_handler = 1 and
re-execute the instruction (yes, SIGTRAP, but this doesn't matter).
When the task hits this bp again we skip handler_chain() because it
was already reported.

Yes? If yes, I don't think this can work. Suppose that the task
dequeues a signal before it returns to the usermode to re-execute
and enters the signal handler which can hit another uprobe.

And this can race with uprobe_register() afaics.

Oleg.