linux-kernel - Re: [PATCH v1 0/5] Implement livepatch on PPC32

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Date:   Mon, 13 Dec 2021 13:54:10 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v1 0/5] Implement livepatch on PPC32

On Mon, 13 Dec 2021 17:50:52 +0000
Christophe Leroy <christophe.leroy@...roup.eu> wrote:

> @@ -958,6 +942,12 @@ unsigned long prepare_ftrace_return(unsigned long 
> parent, unsigned long ip,
>   out:
>   	return parent;
>   }
> +
> +void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
> +		       struct ftrace_ops *op, struct ftrace_regs *fregs)
> +{
> +	prepare_ftrace_return(ip, kernel_stack_pointer(&fregs->regs), 0);
> +}

I have for powerpc prepare_ftrace_return as:


unsigned long prepare_ftrace_return(unsigned long parent, unsigned long ip,
                                                unsigned long sp)
{
        unsigned long return_hooker;

        if (unlikely(ftrace_graph_is_dead()))
                goto out;

        if (unlikely(atomic_read(&current->tracing_graph_pause)))
                goto out;

        return_hooker = ppc_function_entry(return_to_handler);

        if (!function_graph_enter(parent, ip, 0, (unsigned long *)sp))
                parent = return_hooker;
out:
        return parent;
}

Which means you'll need different parameters to it than what x86 has, which
has the prototype of:

void prepare_ftrace_return(unsigned long ip, unsigned long *parent,
			   unsigned long frame_pointer)

and it does not use the frame_pointer for this case, which is why it is
zero.

For powerpc though, it uses the stack pointer, so you parameters are
incorrect. Looks like it should be:

	prepare_ftrace_return(parent_ip, ip, kernel_stack_pointer(&fregs->regs));

And that will likely not be enough. I'll need to update the ctr register,
as that is where the return address is saved. So you'll probably need it to be:

void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
		       struct ftrace_ops *op, struct ftrace_regs *fregs)
{
	unsigned long parent;

	parent = prepare_ftrace_return(parent_ip, ip, kernel_stack_pointer(&fregs->regs));
	fregs->regs.ctr = parent;
}



-- Steve