From: Jiri Olsa <olsajiri@gmail.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
Steven Rostedt <rostedt@goodmis.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
Date: Fri, 9 Jun 2023 09:44:36 -0700 [thread overview]
Message-ID: <ZINW9FqIoja76DRa@krava> (raw)
In-Reply-To: <ZILhqvrjeFIPHauy@FVFF77S0Q05N>
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@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).
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
next prev parent reply other threads:[~2023-06-09 16:44 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-08 21:26 [PATCH RFC] ftrace: Show all functions with addresses in available_filter_functions_addrs Jiri Olsa
2023-06-08 22:43 ` Andrii Nakryiko
2023-06-08 23:27 ` Steven Rostedt
2023-06-08 23:55 ` Andrii Nakryiko
2023-06-09 0:13 ` Jiri Olsa
2023-06-09 8:24 ` Mark Rutland
2023-06-09 16:44 ` Jiri Olsa [this message]
2023-06-09 17:12 ` Steven Rostedt
2023-06-09 18:29 ` Andrii Nakryiko
2023-06-09 20:37 ` Jiri Olsa
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