Re: [PATCH v2] tracing: Optimize event type allocation with IDA

[Zheng Yejian]

On Tue, 22 Nov 2022 22:32:58 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Wed, 23 Nov 2022 11:18:06 +0800
> Zheng Yejian <zhengyejian1@xxxxxxxxxx> wrote:
>
> > But Yujie Liu <yujie.liu@xxxxxxxxx> reported a problem that highly
> > reproducible after applying this patch:
> > Link: https://lore.kernel.org/lkml/54f23c9c-97ae-e326-5873-bfa5d2c81f52@xxxxxxxxx/
> >
> > So please DO NOT apply this patch before I find what happened about it.
>
> I know what the issue is.
>
> The current way of assigning types is to always increment. And not to
> reuse until it fills up. And even then, it looks for the next available
> number.
>
> I'm guessing the IDA will reuse a number as soon as it is freed. This

Yes, it is.

> may also have uncovered a bug, as in reality, we must actually clear
> the tracing buffers every time a number is reused.

Thanks for your explanation!

It seems almost the case, and with current way of assigning types, this
problem maybe also happend when reuse type id, am I right?

But instead of clear tracing buffers, would it be better to just mark
that record invalid if we had a way of knowing that the format had changed?

>
> What happens is that the type number is associated to a print format.
> That is, the raw data is tagged with the type. This type maps to how to
> parse the raw data. If you have a kprobe, it creates a new type number.
> If you free it, and create another one. With the IDA, it is likely to
> reassign the previously freed number to a new probe.
>
> To explain this better, let's look at the following scenario:
>
>  echo 'p:foo val=$arg1:u64' > kprobe_events
>  echo 1 > events/kprobes/foo/enable
>  sleep 1
>  echo 0 > events/kprobes/foo/enable
>
>  echo 'p:bar val=+0($arg1):string' > kprobe_events
>
>  # foo kprobe is deleted and bar is created and
>  # with IDA, bar has the same number for type as foo
>
>  cat trace
>
> When you read the trace, it will see a binary blob representing an
> event and marked with a type. Although the event was foo, it will now
> map it to bar. And it will read foo's $arg1:u64 as bar's
> +0($arg1):string, and will crash.
>
> -- Steve