linux-kernel - Re: [PATCH] x86/unwind/orc: unwind ftrace trampolines with correct orc

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Date:   Wed, 17 Aug 2022 22:28:36 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Chen Zhongjin <chenzhongjin@...wei.com>
Cc:     <linux-kernel@...r.kernel.org>, <jpoimboe@...nel.org>,
        <peterz@...radead.org>, <tglx@...utronix.de>, <mingo@...hat.com>,
        <bp@...en8.de>, <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        <hpa@...or.com>
Subject: Re: [PATCH] x86/unwind/orc: unwind ftrace trampolines with correct
 orc

On Thu, 18 Aug 2022 09:55:25 +0800
Chen Zhongjin <chenzhongjin@...wei.com> wrote:


>  arch/x86/kernel/unwind_orc.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
> index 38185aedf7d1..a938c5d0ed6f 100644
> --- a/arch/x86/kernel/unwind_orc.c
> +++ b/arch/x86/kernel/unwind_orc.c
> @@ -93,22 +93,25 @@ static struct orc_entry *orc_find(unsigned long ip);
>  static struct orc_entry *orc_ftrace_find(unsigned long ip)
>  {
>  	struct ftrace_ops *ops;
> -	unsigned long caller;
> +	unsigned long tramp_addr, offset;
>  
>  	ops = ftrace_ops_trampoline(ip);
>  	if (!ops)
>  		return NULL;
>  

Now if this is that unlikely recursion mentioned below then ops->trampoline
will be NULL, and if we do that offset addition, it will be incorrect.

Perhaps we should add here:

	if (!ops->trampoline)
		return NULL;


Let's add some comments.

	/* Set tramp_addr to the start of the code copied by the trampoline */

>  	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS)
> -		caller = (unsigned long)ftrace_regs_call;
> +		tramp_addr = (unsigned long)ftrace_regs_caller;
>  	else
> -		caller = (unsigned long)ftrace_call;
> +		tramp_addr = (unsigned long)ftrace_caller;
> +

	/* Now place tramp_addr to the location within the trampoline ip is at */

> +	offset = ip - ops->trampoline;
> +	tramp_addr += offset;
>  
>  	/* Prevent unlikely recursion */
> -	if (ip == caller)
> +	if (ip == tramp_addr)
>  		return NULL;
>  
> -	return orc_find(caller);
> +	return orc_find(tramp_addr);
>  }
>  #else
>  static struct orc_entry *orc_ftrace_find(unsigned long ip)