Re: [PATCH v14 03/19] function_graph: Pass ftrace_regs to entryfunc

Steven Rostedt
2024-09-30T18:46:14Z

On Tue, 17 Sep 2024 09:26:25 +0100
Will Deacon <will@kernel.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@kernel.org> wrote:
> >
> > > From: Masami Hiramatsu (Google) <mhiramat@kernel.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.

>
> 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