linux-kernel - Re: [PATCH v18 01/17] fgraph: Pass ftrace

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Message-ID: <20241031155324.108ed8ef@gandalf.local.home>
Date: Thu, 31 Oct 2024 15:53:24 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Florent Revest
 <revest@...omium.org>, linux-trace-kernel@...r.kernel.org, LKML
 <linux-kernel@...r.kernel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,
 bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Jiri Olsa
 <jolsa@...nel.org>, Alan Maguire <alan.maguire@...cle.com>, Mark Rutland
 <mark.rutland@....com>, linux-arch@...r.kernel.org, Catalin Marinas
 <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Huacai Chen
 <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, Michael Ellerman
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 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
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 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v18 01/17] fgraph: Pass ftrace_regs to entryfunc

On Sat, 26 Oct 2024 13:35:30 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Pass ftrace_regs to the fgraph_ops::entryfunc(). If ftrace_regs is not
> available, it passes a NULL instead. User callback function can access
> some registers (including return address) via this ftrace_regs.
> 
> Note that the ftrace_regs can be NULL when the arch does NOT define:
> HAVE_DYNAMIC_FTRACE_WITH_ARGS or HAVE_DYNAMIC_FTRACE_WITH_REGS.
> More specifically, if HAVE_DYNAMIC_FTRACE_WITH_REGS is defined but
> not the HAVE_DYNAMIC_FTRACE_WITH_ARGS, and the ftrace ops used to
> register the function callback does not set FTRACE_OPS_FL_SAVE_REGS.
> In this case, ftrace_regs can be NULL in user callback.

If HAVE_DYNAMIC_FTRACE_WITH_REGS is defined but not
HAVE_DYNAMIC_FTRACE_WITH_ARGS is not, then the callback will have regs defined.

> @@ -977,7 +980,7 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
>  
>  static struct ftrace_ops graph_ops = {
>  	.func			= ftrace_graph_func,
> -	.flags			= FTRACE_OPS_GRAPH_STUB,
> +	.flags			= FTRACE_OPS_GRAPH_STUB | FTRACE_OPS_FL_SAVE_ARGS,

Enabling FTRACE_OPS_FL_SAVE_ARGS will pass full regs in that case. Are you
just saying in the change log that this is what you did? As it currently
reads, it sounds like a fgraph user needs to add FTRACE_OPS_FL_SAVE_REGS??

-- Steve

>  #ifdef FTRACE_GRAPH_TRAMP_ADDR
>  	.trampoline		= FTRACE_GRAPH_TRAMP_ADDR,
>  	/* trampoline_size is only needed for dynamically allocated tramps */
> @@ -987,7 +990,8 @@ static struct ftrace_ops graph_ops = {
>  void fgraph_init_ops(struct ftrace_ops *dst_ops,
>  		     struct ftrace_ops *src_ops)
>  {
> -	dst_ops->flags = FTRACE_OPS_FL_PID | FTRACE_OPS_GRAPH_STUB;
> +	dst_ops->flags = FTRACE_OPS_FL_PID | FTRACE_OPS_GRAPH_STUB |
> +			 FTRACE_OPS_FL_SAVE_ARGS;
>  
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  	if (src_ops) {