linux-kernel - Re: [PATCH ftrace/core v3 1/3] [BUGFIX]kprobes/ftrace: Recover original IP if pre

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Date:	Fri, 18 Jul 2014 10:47:49 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH ftrace/core v3 1/3] [BUGFIX]kprobes/ftrace: Recover original
 IP if pre_handler doesn't change it

(2014/07/18 3:21), Steven Rostedt wrote:
> This looks to require an acked-by from hpa.

Oh, I see.

> 
> On Tue, 15 Jul 2014 06:00:21 +0000
> Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:
> 
>> Recover original IP register if the pre_handler doesn't change it.
>> Since current kprobes doesn't expect that another ftrace handler
>> may change regs->ip, it sets kprobe.addr + MCOUNT_INSN_SIZE to
>> regs->ip and returns to ftrace.
>> This seems wrong behavior since kprobes can recover regs->ip
>> and safely pass it to other handler.
> 
> s/other/another/
> 
>>
>> This adds a code which recovers original regs->ip passed from
> 
>  "... adds code which recovers the original ..."
> 
>> ftrace right before returning ftrace, so that another ftrace user
> 
>  "... returning to ftrace, ..."

Thanks, I'll update the description. :)

> 
>> can change regs->ip.
> 
> You say BUGFIX in the subject, but as nothing changes the regs->ip
> currently from ftrace, is this currently a real bug?

Hmm, indeed, in-tree ftrace handlers never change regs->ip except
kprobes, and for the out-of-tree modules, we need IPMODIFY flag to
avoid such conflict among them. Thus, we can not solve such problem
only by this patch. OK, I'll drop BUGFIX tag.

Thank you,

> 
> -- Steve
> 
>>
>> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
>> ---
>>  arch/x86/kernel/kprobes/ftrace.c |    9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
>> index 717b02a..5f8f0b3 100644
>> --- a/arch/x86/kernel/kprobes/ftrace.c
>> +++ b/arch/x86/kernel/kprobes/ftrace.c
>> @@ -27,7 +27,7 @@
>>  
>>  static nokprobe_inline
>>  int __skip_singlestep(struct kprobe *p, struct pt_regs *regs,
>> -		      struct kprobe_ctlblk *kcb)
>> +		      struct kprobe_ctlblk *kcb, unsigned long orig_ip)
>>  {
>>  	/*
>>  	 * Emulate singlestep (and also recover regs->ip)
>> @@ -39,6 +39,8 @@ int __skip_singlestep(struct kprobe *p, struct pt_regs *regs,
>>  		p->post_handler(p, regs, 0);
>>  	}
>>  	__this_cpu_write(current_kprobe, NULL);
>> +	if (orig_ip)
>> +		regs->ip = orig_ip;
>>  	return 1;
>>  }
>>  
>> @@ -46,7 +48,7 @@ int skip_singlestep(struct kprobe *p, struct pt_regs *regs,
>>  		    struct kprobe_ctlblk *kcb)
>>  {
>>  	if (kprobe_ftrace(p))
>> -		return __skip_singlestep(p, regs, kcb);
>> +		return __skip_singlestep(p, regs, kcb, 0);
>>  	else
>>  		return 0;
>>  }
>> @@ -71,13 +73,14 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
>>  	if (kprobe_running()) {
>>  		kprobes_inc_nmissed_count(p);
>>  	} else {
>> +		unsigned long orig_ip = regs->ip;
>>  		/* Kprobe handler expects regs->ip = ip + 1 as breakpoint hit */
>>  		regs->ip = ip + sizeof(kprobe_opcode_t);
>>  
>>  		__this_cpu_write(current_kprobe, p);
>>  		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
>>  		if (!p->pre_handler || !p->pre_handler(p, regs))
>> -			__skip_singlestep(p, regs, kcb);
>> +			__skip_singlestep(p, regs, kcb, orig_ip);
>>  		/*
>>  		 * If pre_handler returns !0, it sets regs->ip and
>>  		 * resets current kprobe.
>>
> 
> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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