linux-kernel - Re: [PATCH v14 03/19] function_graph: Pass ftrace

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Message-Id: <20241001105753.723314ca371e9f484c88bcac@kernel.org>
Date: Tue, 1 Oct 2024 10:57:53 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Will Deacon <will@...nel.org>, Catalin Marinas
 <catalin.marinas@....com>, linux-arm-kernel@...ts.infradead.org,
 "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, Florent Revest
 <revest@...omium.org>, linux-trace-kernel@...r.kernel.org, LKML
 <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Arnaldo Carvalho de Melo <acme@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Alan Maguire <alan.maguire@...cle.com>,
 Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v14 03/19] function_graph: Pass ftrace_regs to entryfunc

On Mon, 30 Sep 2024 14:46:59 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue, 17 Sep 2024 09:26:25 +0100
> Will Deacon <will@...nel.org> wrote:
> 
> > On Sun, Sep 15, 2024 at 04:46:14AM -0400, Steven Rostedt wrote:
> > > Can I get an Acked-by from the AARCH64 maintainers for this patch?  
> > 
> > Sorry, I wasn't CC'd on the thread, so I missed this.
> 
> Here's the lore link that starts it all:
> 
>   https://lore.kernel.org/all/172615368656.133222.2336770908714920670.stgit@devnote2/
> 
> There's also a v15:
> 
>    https://lore.kernel.org/linux-trace-kernel/172639136989.366111.11359590127009702129.stgit@devnote2/
> 
> > 
> > > On Fri, 13 Sep 2024 00:08:40 +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.  
> > 
> > Under which circumstances is 'ftrace_regs' NULL?
> 
> When the arch does not define: HAVE_DYNAMIC_FTRACE_WITH_ARGS or
> HAVE_DYNAMIC_FTRACE_WITH_REGS
> 
> If HAVE_DYNAMIC_FTRACE_WITH_REGS is defined but not the WITH_ARGS, and the
> ops used to register the function callback does not set FTRACE_OPS_FL_SAVE_REGS.
> 
> Otherwise it should be set.

I will add this condition in the description in next version.

Thank you,


> 
> > 
> > The arm64 implementation of ftrace_graph_func() is:
> > 
> > > > diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> > > > index a650f5e11fc5..bc647b725e6a 100644
> > > > --- a/arch/arm64/kernel/ftrace.c
> > > > +++ b/arch/arm64/kernel/ftrace.c
> > > > @@ -481,7 +481,25 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
> > > >  void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
> > > >  		       struct ftrace_ops *op, struct ftrace_regs *fregs)
> > > >  {
> > > > -	prepare_ftrace_return(ip, &fregs->lr, fregs->fp);
> > > > +	unsigned long return_hooker = (unsigned long)&return_to_handler;
> > > > +	unsigned long frame_pointer = fregs->fp;  
> > 
> > Which dereferences the unchecked pointer here ^^.
> 
> Just above the visible portion of the patch, we have:
> 
> #ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
>                        struct ftrace_ops *op, struct ftrace_regs *fregs)
> {
> 
> Which means fregs will be available. The "WITH_ARGS" config tells us that
> the arch will supply the function with valid fregs to the ftrace callback
> (which this is).
> 
> -- Steve
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>