From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_2 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2F608C4338F for ; Thu, 12 Aug 2021 03:46:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0BC486104F for ; Thu, 12 Aug 2021 03:46:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233669AbhHLDrP (ORCPT ); Wed, 11 Aug 2021 23:47:15 -0400 Received: from mail.kernel.org ([198.145.29.99]:34278 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229729AbhHLDrO (ORCPT ); Wed, 11 Aug 2021 23:47:14 -0400 Received: from rorschach.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 9BE0760FE6; Thu, 12 Aug 2021 03:46:49 +0000 (UTC) Date: Wed, 11 Aug 2021 23:46:48 -0400 From: Steven Rostedt To: Masami Hiramatsu Cc: "Tzvetomir Stoyanov (VMware)" , linux-trace-devel@vger.kernel.org, linux-kernel@vger.kernel.org, tom.zanussi@linux.intel.com Subject: Re: [PATCH v4] [RFC] trace: Add kprobe on tracepoint Message-ID: <20210811234648.4f847ac2@rorschach.local.home> In-Reply-To: <20210812102735.5ac09a88aa6149a239607fd0@kernel.org> References: <20210811141433.1976072-1-tz.stoyanov@gmail.com> <20210812000343.887f0084ff1c48de8c47ec90@kernel.org> <20210811112249.555463f2@oasis.local.home> <20210812102735.5ac09a88aa6149a239607fd0@kernel.org> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-trace-devel@vger.kernel.org On Thu, 12 Aug 2021 10:27:35 +0900 Masami Hiramatsu wrote: > Let me confirm this, so eprobes can be attached to synthetic event? > IMHO, I rather like to prevent attaching eprobe_event on the other > dynamic events. It makes hard to check when removing the base dynamic > events... > > For the above example, we can rewrite it as below to trace filename > without attaching eprobe_events on the synthetic event. > > echo 'my_open pid_t pid; char file[]' > synthetic_events > > echo 'e:myopen syscalls.sys_enter_open file=+0($filename):ustring' > dynamic_events > echo 'e:myopen_ret syscalls.sys_exit_open ret=$ret' > dynamic_events > > echo 'hist:keys=common_pid:fname=file' > events/eprobes/myopen/trigger > echo 'hist:keys=common_pid:fname=$fname:onmatch(eprobes.myopen).trace(my_open,common_pid,$fname)' > events/eprobes/myopen_ret > The problem is that the above wont work :-( For example, I can use this program: #include #include #include #include static const char *file = "/etc/passwd"; int main (int argc, char **argv) { int fd; fd = open(file, O_RDONLY); if (fd < 0) perror(file); close(fd); return 0; } Which if you do the above, all you'll get from the myopen is "(null)". That's because the "/etc/passwd" is not paged in at the start of the system call, and because tracepoints can not fault, the "ustring" will not be mapped yet, it can not give you the content of the file pointer. This was the entire reason we are working on eprobes to attach to synthetic events in the first place. The trick is to use the synthetic event to pass the filename pointer to the exit of the system call, which the system call itself would map the pointer to "file", and when the eprobe reads it with ":ustring" from the exit of the system call it gets "/etc/passwd" instead of "(null)". Your above example doesn't fix this. -- Steve