Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932968AbYF3WHt (ORCPT ); Mon, 30 Jun 2008 18:07:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763078AbYF3WHk (ORCPT ); Mon, 30 Jun 2008 18:07:40 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:43276 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753558AbYF3WHj (ORCPT ); Mon, 30 Jun 2008 18:07:39 -0400 Subject: Re: ADJ_OFFSET_SS_READ bug? From: john stultz To: Michael Kerrisk Cc: Roman Zippel , lkml , Thomas Gleixner , Ingo Molnar In-Reply-To: <485E0007.2020904@gmail.com> References: <485E0007.2020904@gmail.com> Content-Type: text/plain Date: Mon, 30 Jun 2008 15:07:30 -0700 Message-Id: <1214863650.3143.19.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.12.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3142 Lines: 89 On Sun, 2008-06-22 at 09:32 +0200, Michael Kerrisk wrote: > Roman, John > > John, thanks for ADJ_OFFSET_SS_READ, which fixed my bug report > (http://sourceware.org/bugzilla/show_bug?id=2449, > http://bugzilla.kernel.org/show_bug.cgi?id=6761) > > Roman, thanks for fixing John's fix ;-) > > However, I'm wondering if there is a potential bug in the > implementation of this flag. Note the following definitions > from include/linux/timex.h: > > #define ADJ_OFFSET 0x0001 /* time offset */ > [...] > #define ADJ_OFFSET_SINGLESHOT 0x8001 /* old-fashioned adjtime */ > #define ADJ_OFFSET_SS_READ 0xa001 /* read-only adjtime */ > > > Using the the above value for ADJ_OFFSET_SS_READ, where the bits match those > in ADJ_OFFSET and ADJ_OFFSET_SINGLESHOT, seems unnecessary as far as I can > see. Why was that done? Hrm. My original fix was to use 0x2000, but from the commit Ingo changed it at Ulrich's suggestion. Had something to do with old glibc's doing the right thing? > More to the point, it looks like it creates a bug, since the "read-only > adjtime" triggers the code path for ADJ_OFFSET: > > if (txc->modes) { > ... > if (txc->modes & ADJ_OFFSET) { > if (txc->modes == ADJ_OFFSET_SINGLESHOT) > /* adjtime() is independent from ntp_adjtime() */ > time_adjust = txc->offset; > else > ntp_update_offset(txc->offset); /*XXX*/ > } > if (txc->modes & ADJ_TICK) > tick_usec = txc->tick; > > if (txc->modes & (ADJ_TICK|ADJ_FREQUENCY|ADJ_OFFSET)) > ntp_update_frequency(); /*XXX*/ > } > > Unless I misunderstood something, ADJ_OFFSET_SS_READ causes the code marked > XXX to be executed, but I don't think that is what is desired. Is that true? Yea. That does look like an issue. Thanks for the close inspection and review! Sort of a quick off the cuff patch, but does the following look like the right fix to you? Roman: your thoughts? Signed-off-by: John Stultz diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c index 5125ddd..7842a8d 100644 --- a/kernel/time/ntp.c +++ b/kernel/time/ntp.c @@ -379,13 +379,14 @@ int do_adjtimex(struct timex *txc) if (txc->modes == ADJ_OFFSET_SINGLESHOT) /* adjtime() is independent from ntp_adjtime() */ time_adjust = txc->offset; - else + else if (txc->modes != ADJ_OFFSET_SS_READ) ntp_update_offset(txc->offset); } if (txc->modes & ADJ_TICK) tick_usec = txc->tick; - if (txc->modes & (ADJ_TICK|ADJ_FREQUENCY|ADJ_OFFSET)) + if ((txc->modes & (ADJ_TICK|ADJ_FREQUENCY|ADJ_OFFSET)) && + (txc->modes != ADJ_OFFSET_SS_READ)) ntp_update_frequency(); } -- 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/