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Re: [RFC 5/5 v2] uprobes: add global breakpoints
- From: Oleg Nesterov <oleg at redhat dot com>
- To: Sebastian Andrzej Siewior <bigeasy at linutronix dot de>
- Cc: Peter Zijlstra <a dot p dot zijlstra at chello dot nl>, linux-kernel at vger dot kernel dot org, x86 at kernel dot org, Arnaldo Carvalho de Melo <acme at ghostprotocols dot net>, Srikar Dronamraju <srikar at linux dot vnet dot ibm dot com>, Ananth N Mavinakaynahalli <ananth at in dot ibm dot com>, stan_shebs at mentor dot com, gdb-patches at sourceware dot org
- Date: Wed, 22 Aug 2012 15:48:37 +0200
- Subject: Re: [RFC 5/5 v2] uprobes: add global breakpoints
- References: <1344355952-2382-1-git-send-email-bigeasy@linutronix.de> <1344355952-2382-6-git-send-email-bigeasy@linutronix.de> <1344857686.31459.25.camel@twins> <20120821194200.GA32293@linutronix.de>
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.