Re: [PATCH] kprobes/x86: Use the current code when modified by Kprobes using ftrace

[Petr Mladek]

On Mon 2015-02-02 15:57:20, Petr Mladek wrote:
> On Mon 2015-02-02 17:40:10, Masami Hiramatsu wrote:
> > (2015/01/31 0:45), 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().
> > 
> > Good catch!
> > 
> > > This patch makes Kprobe to use the current code when it is modified
> > > by ftrace. It is not the original one but it is a valid instruction
> > > of the same length. Therefore it is perfectly fine for the check.
> > 
> > Hmm, but there is a chance we can hit it in the period of replacing code.
> > This means we'll see 5byte nop but started with int3. (see P6_NOP5 in
> > arch/x86/include/asm/nops.h)
> > So, I think we'd better return a special buffer which has a 5byte NOP.
> 
> Good point! I though that we were synchronized against ftrace by
> text_mutex and kprobe_mutex but we were not. Ftrace does not take
> text_mutex and it can modify the ftrace location also from other
> reasons, e.g. to enable tracing.
> 
> Therefore we need to always check if the ftrace location is included
> in the helper buffer and replace it by the 5-byte nop. It need not be
> on the beginning of the instruction if mcount is used.

Ah, I thought that we needed to detect the ftrace location anywhere
in the insn buffer because we wanted a valid code there. But it seems
that we are always interested only into the first instruction.
I hope that I get it right. If yes, it will be easy in the end.

Best Regards,
Petr

> I am going to prepare v2 of the patch.
> 
> Best Regards,
> Petr
> 
> 
> > Thank you for reporting!
> > 
> > > 
> > > 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.
> > > 
> > > Signed-off-by: Petr Mladek <pmladek@xxxxxxx>
> > > ---
> > >  arch/x86/kernel/kprobes/core.c | 10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > > index 98f654d466e5..ef321caae3ba 100644
> > > --- a/arch/x86/kernel/kprobes/core.c
> > > +++ b/arch/x86/kernel/kprobes/core.c
> > > @@ -225,8 +225,14 @@ __recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr)
> > >  	struct kprobe *kp;
> > >  
> > >  	kp = get_kprobe((void *)addr);
> > > -	/* There is no probe, return original address */
> > > -	if (!kp)
> > > +	/*
> > > +	 * Use the current code if it is not modified by Kprobe
> > > +	 * or when the Kprobe is using ftrace. In the second case
> > > +	 * we do not have any information about the original code
> > > +	 * but it is not a problem. Ftrace has put there a valid
> > > +	 * instruction of the same length.
> > > +	 */
> > > +	if (!kp || kprobe_ftrace(kp))
> > >  		return addr;
> > >  
> > >  	/*
> > > 
> > 
> > 
> > -- 
> > Masami HIRAMATSU
> > Software Platform Research Dept. Linux Technology Research Center
> > Hitachi, Ltd., Yokohama Research Laboratory
> > E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
> > 
> > 
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