linux-kernel - Re: [PATCH bpf-next v2 04/18] ftrace: add reset_ftrace_direct

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Message-ID: <CADxym3YKAW=xt6vGfHLH+tdZCp6iV5va+WkCiS97cokSpuOXiQ@mail.gmail.com>
Date: Tue, 8 Jul 2025 09:26:01 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: alexei.starovoitov@...il.com, jolsa@...nel.org, bpf@...r.kernel.org, 
	Menglong Dong <dongml2@...natelecom.cn>, Mark Rutland <mark.rutland@....com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 04/18] ftrace: add reset_ftrace_direct_ips

On Tue, Jul 8, 2025 at 2:52 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri, 4 Jul 2025 09:54:52 +0800
> Menglong Dong <menglong8.dong@...il.com> wrote:
>
> > > What exactly do you mean by "reset"?
> >
> > It means to reset the filter hash of the ftrace_ops to ips. In
> > the origin logic, the filter hash of a direct ftrace_ops will not
> > be changed. However, in the tracing-multi case, there are
> > multi functions in the filter hash and can change. This function
> > is used to change the filter hash of a direct ftrace_ops.
>
> The above still doesn't make sense to me.
>
> Can you explain more what exactly you are doing at a higher level? To
> me "reset" means to set back to what it originally was (which usually
> is zero or nothing).

Yeah, with pleasure. When we need to update the functions to filter
for a ftrace_ops, we can use ftrace_set_filter_ips(), which is able to
add, remove and reset the functions to filter.

However, we don't have a function to do similar things for a direct
ftrace_ops. What reset_ftrace_direct_ips() do is the same as
ftrace_set_filter_ips() when the "reset" argument of it is 1, and that's
why I call it "reset". Or we can name it something else, such as "update"?

The use case is for the global trampoline. BPF global trampoline has
a direct ftrace_ops. When we attach new tracing-multi progs to the
new kernel functions, we need to add these functions to the ftrace_ops
of the bpf global trampoline.

The "reset_ftrace_direct_ips" can do both adding and removing things
for the direct ftrace_ops. We will get all the functions for the global
trampoline, and "reset" the functions to filter of the ftrace_ops to them.

Hoping I expressed it clearly :/

Thanks!
Menglong Dong

>
> -- Steve