Re: [PATCH tip/master v4] kprobes: extable: Identify kprobes' insn-slots as kernel text area

Masami Hiramatsu
2017-01-10T01:11:13Z

On Mon, 9 Jan 2017 11:36:48 -0600
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Sun, Jan 08, 2017 at 11:58:09PM +0900, Masami Hiramatsu wrote:
> > Make __kernel_text_address()/kernel_text_address() returns
> > true if the given address is on a kprobe's instruction slot,
> > which is generated by kprobes as a trampoline code.
> > This can help stacktraces to determine the address is on a
> > text area or not.
> >
> > To implement this without any sleep in is_kprobe_*_slot(),
> > this also modify insn_cache page list as a rcu list. It may
> > increase processing deley (not processing time) for garbage
> > slot collection, because it requires to wait an additional
> > rcu grance period when freeing a page from the list.
> > However, since it is not a hot path, we may not take care of it.
> >
> > Note: this can give a small overhead to stack unwinders because
> > this adds 2 checks in __kernel_text_address(). However, the
> > impact should be very small, kprobe_insn_pages list has 1 entry
> > per 256 probes(on x86, on arm/arm64 it will be 1024 probes),
> > and kprobe_optinsn_pages has 1 entry per 32 probes(on x86).
> > In most use cases, the number of kprobe events may be less
> > than 20, this means is_kprobe_*_slot() will check just 1 entry.
> >
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
>
> Thanks for doing this Masami! I verified that it works:
>
> Tested-by: Josh Poimboeuf <jpoimboe@redhat.com>

Thanks!

>
> I suspect that BPF generated code also has the same issue, so I'll leave
> the unwinder warning disabled for now.
>
> BTW, I think we'll have more problems with generated code if/when we
> move to an x86 DWARF unwinder, because it won't have any idea how to
> unwind past generated code. Long term I wonder if it would make sense
> to create some kind of framework for creating or registering generated
> code, so we can solve these types of problems in a single place.

Agreed. I think that can also help us to make the generated code RO :).

Thank you,

--
Masami Hiramatsu <mhiramat@kernel.org>