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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 ED0EAC48BD4 for ; Tue, 25 Jun 2019 12:03:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CB558208CA for ; Tue, 25 Jun 2019 12:03:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728595AbfFYMDx (ORCPT ); Tue, 25 Jun 2019 08:03:53 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:42503 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726628AbfFYMDw (ORCPT ); Tue, 25 Jun 2019 08:03:52 -0400 Received: from localhost ([127.0.0.1] helo=vostro.local) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1hfkAm-0006Fr-Ta; Tue, 25 Jun 2019 14:03:41 +0200 From: John Ogness To: Sergey Senozhatsky Cc: Petr Mladek , linux-kernel@vger.kernel.org, Peter Zijlstra , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Andrea Parri , Thomas Gleixner , Sergey Senozhatsky Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation References: <20190607162349.18199-1-john.ogness@linutronix.de> <20190607162349.18199-2-john.ogness@linutronix.de> <20190618045117.GA7419@jagdpanzerIV> <87imt2bl0k.fsf@linutronix.de> <20190625064543.GA19050@jagdpanzerIV> <20190625071500.GB19050@jagdpanzerIV> <875zoujbq4.fsf@linutronix.de> <20190625090620.wufhvdxxiryumdra@pathway.suse.cz> <20190625100322.GD532@jagdpanzerIV> Date: Tue, 25 Jun 2019 14:03:39 +0200 In-Reply-To: <20190625100322.GD532@jagdpanzerIV> (Sergey Senozhatsky's message of "Tue, 25 Jun 2019 19:03:22 +0900") Message-ID: <87woh9hnxg.fsf@linutronix.de> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019-06-25, Sergey Senozhatsky wrote: >>>> In vprintk_emit(), are we going to always reserve 1024-byte >>>> records, since we don't know the size in advance, e.g. >>>> >>>> printk("%pS %s\n", regs->ip, current->name) >>>> prb_reserve(&e, &rb, ????); >>>> >>>> or are we going to run vscnprintf() on a NULL buffer first, >>>> then reserve the exactly required number of bytes and afterwards >>>> vscnprintf(s) -> prb_commit(&e)? >>> >>> (As suggested by Petr) I want to use vscnprintf() on a NULL >>> buffer. However, a NULL buffer is not sufficient because things like the >>> loglevel are sometimes added via %s (for example, in /dev/kmsg). So >>> rather than a NULL buffer, I would use a small buffer on the stack >>> (large enough to store loglevel/cont information). This way we can use >>> vscnprintf() to get the exact size _and_ printk_get_level() will see >>> enough of the formatted string to parse what it needs. >> >> vscnprintf() with NULL pointer is perfectly fine. Only the formatted >> string has variable size. > > Yeah, that should work. Probably. Can't think of any issues, except > for increased CPU usage. Some sprintf() format specifiers are heavier > than the rest (pS/pF on ia64/ppc/hppa). > > OK, very theoretically. > > There is a difference. > > Doing "sz = vscprintf(NULL, msg); vscnprintf(buf, sz, msg)" for > msg_print_text() and msg_print_ext_header() was safe, because the > data - msg - would not change under us, we would work with logbuf > records, IOW with data which is owned by printk() and printk only. > > But doing > sz = vcsprintf(NULL, "xxx", random_pointer); > if ((buf = prb_reserve(... sz))) { > vscnprintf(buf, sz, "xxx", random_pointer); > prb_commit(...); > } > > might have different outcome sometimes. We probably (!!!) can have > some race conditions. The problem is that, unlike msg_print_text() > and msg_print_ext_header(), printk() works with pointers which it > does not own nor control. IOW within single printk() we will access > some random kernel pointers, then do prb stuff, then access those > same pointers, expecting that none of them will ever change their > state. A very simple example > > printk("Comm %s\n", current->comm) > > Suppose printk on CPU0 and ia64_mca_modify_comm on CPU1 > > CPU0 CPU1 > printk(...) > sz = vscprintf(NULL, "Comm %s\n", current->comm); > ia64_mca_modify_comm() > snprintf(comm, sizeof(comm), "%s %d", current->comm, previous_current->pid); > memcpy(current->comm, comm, sizeof(current->comm)); > if ((buf = prb_reserve(... sz))) { > vscnprintf(buf, "Comm %s\n", current->comm); > ^^^^^^^^^^^^^^ ->comm has changed. > Nothing critical, we > should not corrupt > anything, but we will > truncate ->comm if its > new size is larger than > what it used to be when > we did vscprintf(NULL). > prb_commit(...); > } > > Probably there can be other examples. This is a very good point, and quite important. It is not acceptable if some crash output is cut off because of this effect. In my v1 rfc series, I avoided this issue by having a separate dedicated ringbuffer (rb_sprintf) that was used to allocate a temporary max-size (2KB) buffer for sprinting to. Then _that_ was used for the real ringbuffer input (strlen, prb_reserve, memcpy, prb_commit). That would still be the approach of my choice. John Ogness