linux-kernel - Re: [PATCH RFC] ftrace: Show all functions with addresses in available_filter_functions

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Date:   Fri, 9 Jun 2023 09:44:36 -0700
From:   Jiri Olsa <olsajiri@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Jackie Liu <liu.yun@...ux.dev>
Subject: Re: [PATCH RFC] ftrace: Show all functions with addresses in
 available_filter_functions_addrs

On Fri, Jun 09, 2023 at 09:24:10AM +0100, Mark Rutland wrote:
> 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@...dmis.org> wrote:
> > > On Thu, 8 Jun 2023 15:43:03 -0700 Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> > > > On Thu, Jun 8, 2023 at 2:26 PM Jiri Olsa <jolsa@...nel.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).

so we need that for kprobe_multi bpf link, which is based on fprobe,
and that uses ftrace_set_filter_ips to setup the ftrace_ops filter

and ftrace_set_filter_ips works fine with ip address being the address
of the patched instruction (it's matched in ftrace_location)

but right, I did not realize this.. it might cause confusion if people
don't know it's patch-side addresses..  not sure if there's easy way to
get real function address out of rec->ip, but it will also get more
complicated on x86 when IBT is enabled, will check

or we could just use patch-side addresses and reflect that in the file's
name like 'available_filter_functions_patch_addrs' .. it's already long
name ;-)

jirka