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

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Message-ID: <20220818095944.5fbe3e8e@gandalf.local.home>
Date:   Thu, 18 Aug 2022 09:59:44 -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 11:42:17 +0800
Chen Zhongjin <chenzhongjin@...wei.com> wrote:

> Thanks for review!
> 
> 
> On 2022/8/18 10:28, Steven Rostedt wrote:
> > 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;  
> 
> I think when this will return NULL and then stop at orc_find:`if (ip == 
> 0)` and return null_orc_entry.
> 

Duh, you're correct. I wasn't paying attention to how we acquired ops. Yes,
if ops->trampoline is NULL, then it will never be returned by
ftrace_ops_trampoline().
 
> >
> > Let's add some comments.  
> 
> Makes sense.
> 
> If the above explanation logic is fine, I'll add this comment and send v2.
> 

Yes, just add the comments for v2.

Thanks,

-- Steve