Re: [PATCH v2] kprobes/x86: Use 5-byte NOP when the code might be modified by ftrace

[Petr Mladek]

On Wed 2015-02-18 22:22:05, Ingo Molnar wrote:
> 
> * Petr Mladek <pmladek@xxxxxxx> wrote:
> 
> > On Tue 2015-02-03 16:41:39, Masami Hiramatsu wrote:
> > > (2015/02/03 2:48), Petr Mladek 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.
> > > > 
> > > > There is a bug that __recover_probed_insn() tries to restore
> > > > the original code even for Kprobes using the ftrace framework.
> > > > But in this case, the opcode is not stored. See the difference
> > > > between arch_prepare_kprobe() and arch_prepare_kprobe_ftrace().
> > > > The opcode is stored by arch_copy_kprobe() only from
> > > > arch_prepare_kprobe().
> > > > 
> > > > This patch makes Kprobe to use the ideal 5-byte NOP when the code
> > > > can be modified by ftrace. It is the original instruction, see
> > > > ftrace_make_nop() and ftrace_nop_replace().
> > > > 
> > > > Note that we always need to use the NOP for ftrace locations. Kprobes
> > > > do not block ftrace and the instruction might get modified at anytime.
> > > > It might even be in an inconsistent state because it is modified step
> > > > by step using the int3 breakpoint.
> > > > 
> > > > The patch also fixes indentation of the touched comment.
> > > > 
> > > > Note that I found this problem when playing with Kprobes. I did it
> > > > on x86_64 with gcc-4.8.3 that supported -mfentry. I modified
> > > > samples/kprobes/kprobe_example.c and added offset 5 to put
> > > > the probe right after the fentry area:
> > > > 
> > > > --- cut ---
> > > >  static struct kprobe kp = {
> > > >  	.symbol_name	= "do_fork",
> > > > +	.offset = 5,
> > > >  };
> > > > --- cut ---
> > > > 
> > > > Then I was able to load kprobe_example before jprobe_example
> > > > but not the other way around:
> > > > 
> > > > $> modprobe jprobe_example
> > > > $> modprobe kprobe_example
> > > > modprobe: ERROR: could not insert 'kprobe_example': Invalid or incomplete multibyte or wide character
> > > > 
> > > > It did not make much sense and debugging pointed to the bug
> > > > described above.
> > > > 
> > > 
> > > This looks good to me :)
> > > 
> > > Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> > > 
> > > Ingo, could you merge this as an urgent fix?
> > 
> > Please, wait a bit, see below.
> 
> Please [re]send the final patch with Masami's Ack added and 
> me Cc:-ed.

The other problem was solved and acked in a separate patch. I'll rather
send it this way. I am sorry for the confusion.

Best Regards,
Petr
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