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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Tue, 13 Jun 2023 15:44:48 +0200
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Yonghong Song <yhs@meta.com>, Jiri Olsa <olsajiri@gmail.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Jackie Liu <liu.yun@linux.dev>
Subject: Re: [PATCHv2] ftrace: Show all functions with addresses in
 available_filter_functions_addrs
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On Tue, Jun 13, 2023 at 09:36:06AM -0400, Steven Rostedt wrote:
> On Mon, 12 Jun 2023 22:04:28 -0700
> Yonghong Song <yhs@meta.com> wrote:
> 
> > Thanks for explanation! It would be great if we can put more details in
> > this email into the commit message!
> 
> I agree.
> 
> This is the patch I just pulled into my queue:

great, thanks

jirka

> 
> From: Jiri Olsa <jolsa@kernel.org>
> Date: Sun, 11 Jun 2023 15:00:29 +0200
> Subject: [PATCH] ftrace: Show all functions with addresses in
>  available_filter_functions_addrs
> 
> Adding new available_filter_functions_addrs file that shows all available
> functions (same as available_filter_functions) together with addresses,
> like:
> 
>   # cat available_filter_functions_addrs | head
>   ffffffff81000770 __traceiter_initcall_level
>   ffffffff810007c0 __traceiter_initcall_start
>   ffffffff81000810 __traceiter_initcall_finish
>   ffffffff81000860 trace_initcall_finish_cb
>   ...
> 
> Note displayed address is the patch-site address and can differ from
> /proc/kallsyms address.
> 
> It's useful to have address avilable for traceable symbols, so we don't
> need to allways cross check kallsyms with available_filter_functions
> (or the other way around) and have all the data in single file.
> 
> For backwards compatibility reasons we can't change the existing
> available_filter_functions file output, but we need to add new file.
> 
> The problem is that we need to do 2 passes:
> 
>  - through available_filter_functions and find out if the function is traceable
>  - through /proc/kallsyms to get the address for traceable function
> 
> Having available_filter_functions symbols together with addresses allow
> us to skip the kallsyms step and we are ok with the address in
> available_filter_functions_addr not being the function entry, because
> kprobe_multi uses fprobe and that handles both entry and patch-site
> address properly.
> 
> We have 2 interfaces how to create kprobe_multi link:
> 
>   a) passing symbols to kernel
> 
>      1) user gathers symbols and need to ensure that they are
>         trace-able -> pass through available_filter_functions file
> 
>      2) kernel takes those symbols and translates them to addresses
>         through kallsyms api
> 
>      3) addresses are passed to fprobe/ftrace through:
> 
>          register_fprobe_ips
>          -> ftrace_set_filter_ips
> 
>   b) passing addresses to kernel
> 
>      1) user gathers symbols and needs to ensure that they are
>         trace-able -> pass through available_filter_functions file
> 
>      2) user takes those symbols and translates them to addresses
>        through /proc/kallsyms
> 
>      3) addresses are passed to the kernel and kernel calls:
> 
>          register_fprobe_ips
>          -> ftrace_set_filter_ips
> 
> The new available_filter_functions_addrs file helps us with option b),
> because we can make 'b 1' and 'b 2' in one step - while filtering traceable
> functions, we get the address directly.
> 
> Link: https://lore.kernel.org/linux-trace-kernel/20230611130029.1202298-1-jolsa@kernel.org
> 
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Cc: Jackie Liu <liu.yun@linux.dev>
> Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  Documentation/trace/ftrace.rst |  6 ++++++
>  include/linux/ftrace.h         |  1 +
>  kernel/trace/ftrace.c          | 37 ++++++++++++++++++++++++++++++++++
>  3 files changed, 44 insertions(+)
> 
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index df2d3e57a83f..b7308ab10c0e 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -324,6 +324,12 @@ of ftrace. Here is a list of some of the key files:
>  	"set_graph_function", or "set_graph_notrace".
>  	(See the section "dynamic ftrace" below for more details.)
>  
> +  available_filter_functions_addrs:
> +
> +	Similar to available_filter_functions, but with address displayed
> +	for each function. The displayed address is the patch-site address
> +	and can differ from /proc/kallsyms address.
> +
>    dyn_ftrace_total_info:
>  
>  	This file is for debugging purposes. The number of functions that
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 49f279f4c3a1..8e59bd954153 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -633,6 +633,7 @@ enum {
>  	FTRACE_ITER_MOD		= (1 << 5),
>  	FTRACE_ITER_ENABLED	= (1 << 6),
>  	FTRACE_ITER_TOUCHED	= (1 << 7),
> +	FTRACE_ITER_ADDRS	= (1 << 8),
>  };
>  
>  void arch_ftrace_update_code(int command);
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 764668467155..b24c573934af 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3861,6 +3861,9 @@ static int t_show(struct seq_file *m, void *v)
>  	if (!rec)
>  		return 0;
>  
> +	if (iter->flags & FTRACE_ITER_ADDRS)
> +		seq_printf(m, "%lx ", rec->ip);
> +
>  	if (print_rec(m, rec->ip)) {
>  		/* This should only happen when a rec is disabled */
>  		WARN_ON_ONCE(!(rec->flags & FTRACE_FL_DISABLED));
> @@ -3996,6 +3999,30 @@ ftrace_touched_open(struct inode *inode, struct file *file)
>  	return 0;
>  }
>  
> +static int
> +ftrace_avail_addrs_open(struct inode *inode, struct file *file)
> +{
> +	struct ftrace_iterator *iter;
> +	int ret;
> +
> +	ret = security_locked_down(LOCKDOWN_TRACEFS);
> +	if (ret)
> +		return ret;
> +
> +	if (unlikely(ftrace_disabled))
> +		return -ENODEV;
> +
> +	iter = __seq_open_private(file, &show_ftrace_seq_ops, sizeof(*iter));
> +	if (!iter)
> +		return -ENOMEM;
> +
> +	iter->pg = ftrace_pages_start;
> +	iter->flags = FTRACE_ITER_ADDRS;
> +	iter->ops = &global_ops;
> +
> +	return 0;
> +}
> +
>  /**
>   * ftrace_regex_open - initialize function tracer filter files
>   * @ops: The ftrace_ops that hold the hash filters
> @@ -5916,6 +5943,13 @@ static const struct file_operations ftrace_touched_fops = {
>  	.release = seq_release_private,
>  };
>  
> +static const struct file_operations ftrace_avail_addrs_fops = {
> +	.open = ftrace_avail_addrs_open,
> +	.read = seq_read,
> +	.llseek = seq_lseek,
> +	.release = seq_release_private,
> +};
> +
>  static const struct file_operations ftrace_filter_fops = {
>  	.open = ftrace_filter_open,
>  	.read = seq_read,
> @@ -6377,6 +6411,9 @@ static __init int ftrace_init_dyn_tracefs(struct dentry *d_tracer)
>  	trace_create_file("available_filter_functions", TRACE_MODE_READ,
>  			d_tracer, NULL, &ftrace_avail_fops);
>  
> +	trace_create_file("available_filter_functions_addrs", TRACE_MODE_READ,
> +			d_tracer, NULL, &ftrace_avail_addrs_fops);
> +
>  	trace_create_file("enabled_functions", TRACE_MODE_READ,
>  			d_tracer, NULL, &ftrace_enabled_fops);
>  
> -- 
> 2.39.2
>