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Date:   Fri, 9 Jun 2023 09:24:10 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>, Jiri Olsa <jolsa@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org,
        Jackie Liu <liu.yun@linux.dev>
Subject: Re: [PATCH RFC] ftrace: Show all functions with addresses in
 available_filter_functions_addrs
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On Thu, Jun 08, 2023 at 04:55:40PM -0700, Andrii Nakryiko wrote:
> On Thu, Jun 8, 2023 at 4:27 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Thu, 8 Jun 2023 15:43:03 -0700 Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> > > On Thu, Jun 8, 2023 at 2:26 PM Jiri Olsa <jolsa@kernel.org> wrote:
 
> There are BPF tools that allow user to specify regex/glob of kernel
> functions to attach to. This regex/glob is checked against
> available_filter_functions to check which functions are traceable. All
> good. But then also it's important to have corresponding memory
> addresses for selected functions (for many reasons, e.g., to have
> non-ambiguous and fast attachment by address instead of by name, or
> for some post-processing based on captured IP addresses, etc). And
> that means that now we need to also parse /proc/kallsyms and
> cross-join it with data fetched from available_filter_functions.
> 
> All this is unnecessary if avalable_filter_functions would just
> provide function address in the first place. It's a huge
> simplification. And saves memory and CPU.

Do you need the address of the function entry-point or the address of the
patch-site within the function? Those can differ, and the rec->ip address won't
necessarily equal the address in /proc/kallsyms, so the pointer in
/proc/kallsyms won't (always) match the address we could print for the ftrace site.

On arm64, today we can have offsets of +0, +4, and +8, and within a single
kernel image different functions can have different offsets. I suspect in
future that we may have more potential offsets (e.g. due to changes for HW/SW
CFI).

Thanks,
Mark.