Re: [PATCH] tracepoint: Add tracepoint_probe_register_may_exist() for BPF tracing

On Wed, 7 Jul 2021 17:23:54 -0700
Andrii Nakryiko [off-list ref] wrote:

On Wed, Jul 7, 2021 at 5:05 PM Steven Rostedt [off-list ref] wrote:

quoted

On Wed, 7 Jul 2021 16:49:26 -0700
Andrii Nakryiko [off-list ref] wrote:
 

quoted

As for why the user might need that, it's up to the user and I don't
want to speculate because it will always sound contrived without a
specific production use case. But people are very creative and we try
not to dictate how and what can be done if it doesn't break any
fundamental assumption and safety.  

I guess it doesn't matter, because if they try to do it, the second
attachment will simply fail to attach.
 

But not for the kprobe case.

What do you mean "not for the kprobe case"? What kprobe case?

You attach the same program twice to the same kprobe? Or do you create
two kprobes at the same location?

And it might not always be possible to know that the same BPF program
is being attached. It could be attached by different processes that
re-use pinned program (without being aware of each other). Or it could
be done from some generic library that just accepts prog_fd and
doesn't really know the exact BPF program and whether it was already
attached.

Not sure why it doesn't matter that attachment will fail where it is
expected to succeed. The question is rather why such restriction?

Why is it expected to succeed? It never did. And why such a
restriction? Because it complicates the code, and there's no good use
case to do so. Why complicate something for little reward?

-- Steve