linux-kernel - Re: [PATCH -tip v2 3/3] [BUGFIX] kprobes: Prohibit probing on func_ptr_is_kernel

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Date:	Tue, 5 Nov 2013 07:09:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	x86@...nel.org, lkml <linux-kernel@...r.kernel.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH -tip v2 3/3] [BUGFIX] kprobes: Prohibit probing on
 func_ptr_is_kernel_text


* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 01 Nov 2013 11:25:37 +0000
> Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:
> 
> > Prohibit probing on func_ptr_is_kernel_text().
> > Since the func_ptr_is_kernel_text() is called from
> > notifier_call_chain() which is called from int3 handler,
> > probing it may cause double int3 fault and kernel will
> > reboot.
> > 
> > This happenes when the kernel built with CONFIG_DEBUG_NOTIFIERS=y.
> > 
> > Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de>
> > Cc: Borislav Petkov <bp@...e.de>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > ---
> >  kernel/extable.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/extable.c b/kernel/extable.c
> > index 832cb28..022fb25 100644
> > --- a/kernel/extable.c
> > +++ b/kernel/extable.c
> > @@ -129,7 +129,7 @@ int kernel_text_address(unsigned long addr)
> >   * pointer is part of the kernel text, we need to do some
> >   * special dereferencing first.
> >   */
> > -int func_ptr_is_kernel_text(void *ptr)
> > +int nokprobe func_ptr_is_kernel_text(void *ptr)
> >  {
> >  	unsigned long addr;
> >  	addr = (unsigned long) dereference_function_descriptor(ptr);
> > 
> 
> One thing I worry about the "nokprobe" annotation, is that it moves the 
> location of the function out of local. This function no exists in the 
> section with its users. Same with the debug functions in the other 
> patch.

Well, it's a bit like noinline, that changes the position of the function 
as well. So it's not true that 'noxyz' attributes don't affect function 
placement - they often don't, but some do.

The more important aspect is that 'noprobe' makes it really, really 
apparent what the tag is about, at first sight.

_How_ the 'non probing' is achived is an implementational detail when 
kprobes are enabled: right now it puts a function into a separate section, 
but we could just a much build a list of function names and check against 
it at probe insertion time.

Thanks,

	Ingo
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