linux-kernel - Re: [PATCH v2 1/4] ftrace: Add print_function

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Message-ID: <20250108102443.415495e1@gandalf.local.home>
Date: Wed, 8 Jan 2025 10:24:43 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Alan Maguire <alan.maguire@...cle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, Donglin Peng
 <dolinux.peng@...il.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Sven Schnelle <svens@...ux.ibm.com>, Paul
 Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>, Zheng
 Yejian <zhengyejian@...weicloud.com>, bpf@...r.kernel.org
Subject: Re: [PATCH v2 1/4] ftrace: Add print_function_args()

On Wed, 8 Jan 2025 11:19:11 +0000
Alan Maguire <alan.maguire@...cle.com> wrote:

> >>> +       trace_seq_printf(s, "(");
> >>> +
> >>> +       if (!args)
> >>> +               goto out;
> >>> +       if (lookup_symbol_name(func, name))
> >>> +               goto out;
> >>> +
> >>> +       btf = bpf_get_btf_vmlinux();
> >>> +       if (IS_ERR_OR_NULL(btf))
> >>> +               goto out;  
> >>
> >>
> >> There is no need to the retrieve the BTF of vmlinux, as btf_find_func_proto
> >> will return the correct BTF via its second parameter.  
> > 
> > Good catch! The second parameter of btf_find_func_proto() is output.
> >  
> 
> One thought here - with btf_find_func_proto(), we will try kernel BTF
> and then proceed to module BTF, iterating over all modules to find the
> function prototype. So where we are tracing module functions this could
> get expensive if such a function is frequently encountered, and it also
> opens up the risk that we end up using the wrong function prototype from
> the wrong module that just happens to match on function name.
> 
> So I wonder if we could use the function address to do a more guided
> lookup. Perhaps we could use kallsyms_lookup(), retrieving the
> (potential) module name. Then maybe modify the signature of
> btf_find_func_proto() to take an optional module name parameter to avoid
> iteration? None of this is strictly needed, but it may speed things up a
> bit and give us more accurate parameter info for those few cases with
> name clashes, so could be done as a follow-up if needed. Thanks!

Well, every place this is called, we first get the function name from
kallsyms. Perhaps I can modify the code to get the module name as well, and
if it exists, we can pass that too?

-- Steve