linux-kernel - Re: [PATCH] tracepoints: prevents null probe from being added

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Date:	Thu, 21 Mar 2013 10:45:47 +0900
From:	Keun-O Park <kpark3469@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	keun-o.park@...driver.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracepoints: prevents null probe from being added

On Thu, Mar 21, 2013 at 10:39 AM, Keun-O Park <kpark3469@...il.com> wrote:
> On Thu, Mar 21, 2013 at 8:01 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> On Wed, 2013-03-20 at 14:01 -0400, Mathieu Desnoyers wrote:
>>> * Steven Rostedt (rostedt@...dmis.org) wrote:
>>> > On Wed, 2013-03-20 at 12:18 +0900, kpark3469@...il.com wrote:
>>> > > From: Sahara <keun-o.park@...driver.com>
>>> > >
>>> > > Somehow tracepoint_entry_add/remove_probe functions allow a null probe
>>> > > function.
>>> >
>>> > You actually hit this in practice, or is this just something that you
>>> > observe from code review?
>>> >
>>> > >  Especially on getting a null probe in remove function, it seems
>>> > > to be used to remove all probe functions in the entry.
>>> >
>>> > Hmm, that actually sounds like a feature.
>>>
>>> Yep. It's been a long time since I wrote this code, but the removal code
>>> seems to use NULL probe pointer to remove all probes for a given
>>> tracepoint.
>>>
>>> I'd be tempted to just validate non-NULL probe within
>>> tracepoint_entry_add_probe() and let other sites as is, just in case
>>> anyone would be using this feature.
>>>
>>> I cannot say that I have personally used this "remove all" feature much
>>> though.
>>>
>>
>> I agree. I don't see anything wrong in leaving the null probe feature in
>> the removal code. But updating the add code looks like a proper change.
>>
>> -- Steve
>>
>>
>
> Hello Steve & Mathieu,
> If we want to leave the null probe feature enabled, I think it would
> be better modifying the code like the following for code efficiency.
>
> @@ -112,7 +112,8 @@ tracepoint_entry_add_probe(struct tracepoint_entry *entry,
>         int nr_probes = 0;
>         struct tracepoint_func *old, *new;
>
> -       WARN_ON(!probe);
> +       if (WARN_ON(!probe))
> +               return ERR_PTR(-EINVAL);
>
>         debug_print_probes(entry);
>         old = entry->funcs;
> @@ -152,14 +153,15 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *ent
>
>         debug_print_probes(entry);
>         /* (N -> M), (N > 1, M >= 0) probes */
> -       for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
> -               if (!probe ||
> -                   (old[nr_probes].func == probe &&
> -                    old[nr_probes].data == data))
> -                       nr_del++;
> +       if (probe) {
> +               for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
> +                       if (old[nr_probes].func == probe &&
> +                            old[nr_probes].data == data)
> +                               nr_del++;
> +               }
>         }
>
> -       if (nr_probes - nr_del == 0) {
> +       if (!probe || nr_probes - nr_del == 0) {
>                 /* N -> 0, (N > 1) */
>                 entry->funcs = NULL;
>                 entry->refcount = 0;
>
> Because we know handing over the null probe to
> tracepoint_entry_add_probe is not possible,
> we don't have to check if the probe is null or not within for loop. If

Hmm. I described this wrong. :-(
For code efficiency, I replaced 'checking null probe in every
iteration within for-loop' with 'checking once outside the loop'.

> the probe is null, it's just enough to add !probe in
> 'if(nr_probes-nr_del==0)'. And, with additional if-clause wrapping
> for-loop, falling through for-loop can be prevented when probe is
> null.
>
> @@ -173,8 +172,7 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *entry
>                 if (new == NULL)
>                         return ERR_PTR(-ENOMEM);
>                 for (i = 0; old[i].func; i++)
> -                       if (probe &&
> -                           (old[i].func != probe || old[i].data != data))
> +                       if (old[i].func != probe || old[i].data != data)
>                                 new[j++] = old[i];
>                 new[nr_probes - nr_del].func = NULL;
>                 entry->refcount = nr_probes - nr_del;
>
> We don't have to check the probe here too. We know probe is always true here.
> Thanks.
>
> -- Kpark
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