Re: [PATCH 3/3] early kprobes: x86: don't try to recover ftraced instruction before ftrace get ready.
From: Wang Nan
Date: Tue Mar 03 2015 - 23:41:34 EST
On 2015/3/4 11:36, Masami Hiramatsu wrote:
> (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
>>>> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
>>>> 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);
>>> 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 + * been replaced by a NOP yet.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,
For this specific case, calling __recover_probed_insn() is mandatory for
can_boost(). However, we can disallow early kprobes to be unregistered before
ftrace is ready, and let ftrace fix all inconsistency by calling
kprobe_on_ftrace_get_old_insn(). Which will make things simpler, and constrain
the using scope of kprobes_on_ftrace_initialized to kernel/kprobes.c. The
cost is unable to do smoke test for early ftrace because it will remove all
kprobe before returning. I think it should be acceptable. What do you think?
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