From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AH8x226m/eeYsMGDoZZO/4qzF4EdKhAn6Qe1dzlWYuS2tpDOrR0Y3QQWvcvzS+tOXhHmhon58rxN ARC-Seal: i=1; a=rsa-sha256; t=1516993199; cv=none; d=google.com; s=arc-20160816; b=bKlVTPJLBwWHY+FksfbjR6T6TBZLvio6yL5dASSOgkPJ7y8fQ4J14RKxzclMTnHHI2 37Nn2NkFBJ8mlbioifLmMeCcKcAICrsrgHryKBZtzPByRc4FMmf331J2+pYbdAGLvzBG UOVLpAqZrf3vc+IebCxjqWqxOqJ+kNfndZp0ylorgchRVG9vWFfuPwWoJhXZCWJ0Zlgu iTUyH7NkgiIsszLhpolKgtrzXkxCrvrSKSpBh7EyY1+ljuhysHn3p7Owip7uTzXWi+/r WeLLYB81YRA0m0BiIRwX2eP97fxm7YrDyZ0uS5LSaGOdoimEbVHjN3Pa27XkCKRx0p6h 5h4A== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:cc:to:subject:message-id:date:from :references:in-reply-to:mime-version:dmarc-filter :arc-authentication-results; bh=LBVdTr4rtnqRRFBXX1ky3d+Xq4txTcIoH/cGBjk0Iuk=; b=MaWUCMBZUNr3ibmiqL0Wp8fKy2i7UtT82cifjpQTyZ98oW76nXjd+OZaESM7t87IRq KCqddyK+hVTxdssYJHgFIjfQFAujvftNOYDLtLfymFqio+t1WKZKCvXTkW8S+OIfqF1x H8BsGgcKU9JOQwPsUKGTzc1XU5YPf0hxvWJjfSuz4VwsR0qHfT+cwOtyrNMyPk+gXKBQ KgvSKr8szkcA+U6UHLMYyfJlHatNiJZF+2Ox/gtlllA2TFCmoPRUMPOKkCLXQMKo3mQt REtFDyT17E9LjW2L9LtF5Fj0mg0CZ1LdpaWO0N/CjRnSbSbjOkxjW737atU7ui+3a2D+ 7SVw== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of linux-kselftest-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kselftest-owner@vger.kernel.org Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kselftest-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kselftest-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752246AbeAZS76 (ORCPT ); Fri, 26 Jan 2018 13:59:58 -0500 Received: from mail.kernel.org ([198.145.29.99]:60008 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751861AbeAZS76 (ORCPT ); Fri, 26 Jan 2018 13:59:58 -0500 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 82608217DF Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=luto@kernel.org MIME-Version: 1.0 In-Reply-To: References: <20180126153631.ha7yc33fj5uhitjo@xps> From: Andy Lutomirski Date: Fri, 26 Jan 2018 10:59:36 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: selftests/x86/fsgsbase_64 test problem To: Andy Lutomirski Cc: Dan Rue , Shuah Khan , Ingo Molnar , Dmitry Safonov , Borislav Petkov , "open list:KERNEL SELFTEST FRAMEWORK" , LKML Content-Type: text/plain; charset="UTF-8" Sender: linux-kselftest-owner@vger.kernel.org X-Mailing-List: linux-kselftest@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1590669876861238825?= X-GMAIL-MSGID: =?utf-8?q?1590682660418534163?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Fri, Jan 26, 2018 at 8:22 AM, Andy Lutomirski wrote: > On Fri, Jan 26, 2018 at 7:36 AM, Dan Rue wrote: >> >> We've noticed that fsgsbase_64 can fail intermittently with the >> following error: >> >> [RUN] ARCH_SET_GS(0x0) and clear gs, then schedule to 0x1 >> Before schedule, set selector to 0x1 >> other thread: ARCH_SET_GS(0x1) -- sel is 0x0 >> [FAIL] GS/BASE changed from 0x1/0x0 to 0x0/0x0 >> >> This can be reliably reproduced by running fsgsbase_64 in a loop. i.e. >> >> for i in $(seq 1 10000); do ./fsgsbase_64 || break; done >> >> This problem isn't new - I've reproduced it on latest mainline and every >> release going back to v4.12 (I did not try earlier). This was tested on >> a Supermicro board with a Xeon E3-1220 as well as an Intel Nuc with an >> i3-5010U. >> > > Hmm, I can reproduce it, too. I'll look in a bit. I'm triggering a different error, and I think what's going on is that the kernel doesn't currently re-save GSBASE when a task switches out and that task has save gsbase != 0 and in-register GS == 0. This is arguably a bug, but it's not an infoleak, and fixing it could be a wee bit expensive. I'm not sure what, if anything, to do about this. I suppose I could add some gross perf hackery to the test to detect this case and suppress the error. I can also trigger the problem you're seeing, and I don't know what's up. It may be related to and old problem I've seen that causes signal delivery to sometimes corrupt %gs. It's deterministic, but it depends in some odd way on register state. I can currently reproduce that issue 100% of the time, and I'm trying to see if I can figure out what's happening.