Subject: [PATCH 3/3] early kprobes: x86: don't try to recover ftraced instruction before ftrace get ready.

From: [email-masked] (Masami Hiramatsu)
Date: Wed, 04 Mar 2015 12:36:41 +0900

(2015/03/04 11:24), Wang Nan wrote:
> On 2015/3/4 1:06, Petr Mladek wrote:
>> On Tue 2015-03-03 13:09:05, Wang Nan wrote:
>>> Before ftrace convertin instruction to nop, if an early kprobe is
>>> registered then unregistered, without this patch its first bytes will
>>> be replaced by head of NOP, which may confuse ftrace.
>>>
>>> Actually, since we have a patch which convert ftrace entry to nop
>>> when probing, this problem should never be triggered. Provide it for
>>> safety.
>>>
>>> Signed-off-by: Wang Nan <[email-masked]>
>>> ---
>>>  arch/x86/kernel/kprobes/core.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
>>> index 87beb64..c7d304d 100644
>>> --- a/arch/x86/kernel/kprobes/core.c
>>> +++ b/arch/x86/kernel/kprobes/core.c
>>> @@ -225,6 +225,9 @@ __recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr)
>>>  	struct kprobe *kp;
>>>  	unsigned long faddr;
>>>  
>>> +	if (!kprobes_on_ftrace_initialized)
>>> +		return addr;
>>
>> This is not correct. The function has to return a buffer with the original
>> code also when it is modified by normal kprobes. If it is a normal
>> Kprobe, it reads the current code and replaces the first byte (INT3
>> instruction) with the saved kp->opcode.
>>
>>> +
>>>  	kp = get_kprobe((void *)addr);
>>>  	faddr = ftrace_location(addr);
>>
>> IMHO, the proper fix might be to replace the above line with
>>
>> 	if (kprobes_on_ftrace_initialized)
>> 		faddr = ftrace_location(addr);
>> 	else
>> 		faddr = 0UL;
>>
>> By other words, it might pretend that it is not a ftrace location
>> when the ftrace is not ready yet.
>>
> 
> Thanks for your reply. I'll follow your suggection in my next version. I change
> it as follow to enable the checking.
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 4e3d5a9..3241677 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -234,6 +234,20 @@ __recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr)
> 	 */
> 	if (WARN_ON(faddr && faddr != addr))
> 		return 0UL;
> +
> +	/*
> +	 * If ftrace is not ready yet, pretend this is not an ftrace
> +	 * location, because currently the target instruction has not
> +	 * been replaced by a NOP yet. When ftrace trying to convert
> +	 * it to NOP, kprobe should be notified and the kprobe data
> +	 * should be fixed at that time.
> +	 *
> +	 * Since it is possible that an early kprobe already on that
> +	 * place, don't return addr directly.
> +	 */
> +	if (likely(kprobes_on_ftrace_initialized))
> +		faddr = 0UL;
> +
> 	/*
> 	 * Use the current code if it is not modified by Kprobe
> 	 * and it cannot be modified by ftrace
> 

This is better, but I don't think we need bool XXX_initialized flags
for each subfunctions. Those should be serialized.

Thank you,

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