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

From: [email-masked] (Wang Nan)
Date: Wed, 4 Mar 2015 10:24:55 +0800

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+	 * been replaced by a NOP yet.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

> Or is the code modified another special way when it is a ftrace location but
> ftrace has not been initialized yet?
> 
> Best Regards,
> Petr
> 
>>  	/*
>> -- 
>> 1.8.4
>>
>> --
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