On 04/16, Frederic Weisbecker wrote:
>
> On Sun, Apr 14, 2013 at 09:12:32PM +0200, Oleg Nesterov wrote:
> > ptrace_write_dr7() skips ptrace_modify_breakpoint(disabled => true)
> > unless second_pass, this buys nothing but complicates the code and
> > means that we always do the main loop twice even if "disabled" was
> > never true.
> >
> > The comment says:
> >
> > Don't unregister the breakpoints right-away,
> > unless all register_user_hw_breakpoint()
> > requests have succeeded.
> >
> > I think this logic was always wrong, hw_breakpoint_del() does not
> > free the slot so perf_event_disable() can't hurt.
>
> For the record, I think it was necessary before
> 44234adcdce38f83c56e05f808ce656175b4beeb
> ("hw-breakpoints: Modify breakpoints without unregistering them") because
> modifying a breakpoint implied that the old bp was released and a new one
> was created, opening a little race window in between against concurrent
> breakpoint users.
Aah, thank, I'll update the changelog.
> Acked-by: Frederic Weisbecker <fweis...@gmail.com>
Thanks!
> > old_dr7 = ptrace_get_dr7(thread->ptrace_bps);
> > @@ -651,35 +643,31 @@ restore:
> > bool disabled = !decode_dr7(data, i, &len, &type);
> > struct perf_event *bp = thread->ptrace_bps[i];
> >
> > - if (disabled) {
> > + if (!bp) {
> > + if (disabled)
> > + continue;
> > /*
> > - * Don't unregister the breakpoints right-away, unless
> > - * all register_user_hw_breakpoint() requests have
> > - * succeeded. This prevents any window of opportunity
> > - * for debug register grabbing by other users.
> > + * We should have at least an inactive breakpoint at
> > + * this slot. It means the user is writing dr7 without
> > + * having written the address register first.
> > */
> > - if (!bp || !second_pass)
> > - continue;
> > + rc = -EINVAL;
> > + break;
> > }
> >
> > rc = ptrace_modify_breakpoint(bp, len, type, tsk, disabled);
> > if (rc)
> > break;
>
> It would be nice to warn here:
>
> WARN_ON_ONCE(rc && second_pass);
Well, I disagree.
To clarify, I agree with WARN_ON_ONCE(), but afaics it has nothing to
do with "second_pass",
> And these are indeed supposed
> to.
Indeed, but this is because ptrace_modify_breakpoint() should not fail.
So, what do you think if I change the main loop above
rc = ptrace_modify_breakpoint(...)
- if (rc)
+ if (WARN_ON_ONCE(rc))
break;
instead?
Oleg.
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