Re: [PATCH 1/2] kprobes/x86: Use 5-byte NOP when the code might be modified by ftrace

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2015/02/20 21:52), Ingo Molnar wrote:
> 
> * Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> 
>> (2015/02/20 19:26), Ingo Molnar wrote:
>>>
>>> * Petr Mladek <pmladek@suse.cz> wrote:
>>>
>>>> can_probe() checks if the given address points to the 
>>>> beginning of an instruction. It analyzes all the 
>>>> instructions from the beginning of the function until the 
>>>> given address. The code might be modified by another 
>>>> Kprobe. In this case, the current code is read into a 
>>>> buffer, int3 breakpoint is replaced by the saved opcode 
>>>> in the buffer, and can_probe() analyzes the buffer 
>>>> instead.
>>>>
>>>> [...]
>>>
>>> Had to drop this patch due to build failures on 32-bit x86:
>>>
>>> arch/x86/kernel/kprobes/core.c:258:40: error: ‘MCOUNT_INSN_SIZE’ undeclared (first use in this function)
>>
>> Oops, MCOUNT_INSN_SIZE actually depends on 
>> CONFIG_FUNCTION_TRACER... I think we can use 5 instead of 
>> that since we are copying NOP_ATOMIC5.
> 
> Or just make the define more widely available? It's not 
> like the size changes from disabling the function tracer.

Yeah, it could be.
Actually, to tell the truth, if CONFIG_FUNCTION_TRACER=n,
ftrace_location() always returns 0, so the 2nd memcpy never be
executed (it should be disappeared by optimization).
Thus *this* issue is very local one, and I thought we'd better
fix this locally. :)

Thank you,

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