Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932924AbbBBIkU (ORCPT ); Mon, 2 Feb 2015 03:40:20 -0500 Received: from mail9.hitachi.co.jp ([133.145.228.44]:59865 "EHLO mail9.hitachi.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932802AbbBBIkT (ORCPT ); Mon, 2 Feb 2015 03:40:19 -0500 Message-ID: <54CF37EA.50000@hitachi.com> Date: Mon, 02 Feb 2015 17:40:10 +0900 From: Masami Hiramatsu Organization: Hitachi, Ltd., Japan User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20120614 Thunderbird/13.0.1 MIME-Version: 1.0 To: Petr Mladek Cc: "David S. Miller" , Anil S Keshavamurthy , Ananth N Mavinakayanahalli , Frederic Weisbecker , Steven Rostedt , Jiri Kosina , linux-kernel@vger.kernel.org, Ingo Molnar Subject: Re: [PATCH] kprobes/x86: Use the current code when modified by Kprobes using ftrace References: <1422632700-4342-1-git-send-email-pmladek@suse.cz> In-Reply-To: <1422632700-4342-1-git-send-email-pmladek@suse.cz> Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3328 Lines: 92 (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. 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 > --- > 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@hitachi.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/