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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id D25F0C433F5 for ; Tue, 15 Mar 2022 18:32:56 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 1D7F78D0002; Tue, 15 Mar 2022 14:32:56 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1871F8D0001; Tue, 15 Mar 2022 14:32:56 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 001458D0002; Tue, 15 Mar 2022 14:32:55 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.28]) by kanga.kvack.org (Postfix) with ESMTP id E3A5D8D0001 for ; Tue, 15 Mar 2022 14:32:55 -0400 (EDT) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id ADF2924845 for ; Tue, 15 Mar 2022 18:32:55 +0000 (UTC) X-FDA: 79247467110.02.7E81F8B Received: from out01.mta.xmission.com (out01.mta.xmission.com [166.70.13.231]) by imf20.hostedemail.com (Postfix) with ESMTP id ED3671C001A for ; Tue, 15 Mar 2022 18:32:54 +0000 (UTC) Received: from in02.mta.xmission.com ([166.70.13.52]:43500) by out01.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nUByW-004bza-N6; Tue, 15 Mar 2022 12:32:52 -0600 Received: from ip68-227-174-4.om.om.cox.net ([68.227.174.4]:37806 helo=email.froward.int.ebiederm.org.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nUByV-00DZm9-EB; Tue, 15 Mar 2022 12:32:52 -0600 From: "Eric W. Biederman" To: Miaohe Lin Cc: , , , , Alexey Gladkov References: <20220314064039.62972-1-linmiaohe@huawei.com> <87h78036hl.fsf@email.froward.int.ebiederm.org> <82cf5aa8-a721-3ff3-7b09-54a66da0d506@huawei.com> Date: Tue, 15 Mar 2022 13:32:28 -0500 In-Reply-To: <82cf5aa8-a721-3ff3-7b09-54a66da0d506@huawei.com> (Miaohe Lin's message of "Tue, 15 Mar 2022 20:17:57 +0800") Message-ID: <87lexbyslf.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1nUByV-00DZm9-EB;;;mid=<87lexbyslf.fsf@email.froward.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.174.4;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX18FfFa8rTN4mQyugIlm4nCio3vDraMI37o= X-SA-Exim-Connect-IP: 68.227.174.4 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v2] mm/mlock: fix potential imbalanced rlimit ucounts adjustment X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) X-Stat-Signature: txtpfsnan6qzu9t5p7a7hpwum1mnmmf6 Authentication-Results: imf20.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=xmission.com; spf=pass (imf20.hostedemail.com: domain of ebiederm@xmission.com designates 166.70.13.231 as permitted sender) smtp.mailfrom=ebiederm@xmission.com X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: ED3671C001A X-HE-Tag: 1647369174-279311 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Miaohe Lin writes: > On 2022/3/14 23:21, Eric W. Biederman wrote: >> Miaohe Lin writes: >> >>> user_shm_lock forgets to set allowed to 0 when get_ucounts fails. So >>> the later user_shm_unlock might do the extra dec_rlimit_ucounts. Fix >>> this by resetting allowed to 0. >> >> This fix looks correct. But the ability for people to follow and read >> the code seems questionable. I saw in v1 of this patch Hugh originally >> misread the logic. >> >> Could we instead change the code to leave lock_limit at ULONG_MAX aka >> RLIM_INFINITY, leave initialized to 0, and not even need a special case >> of RLIM_INFINITY as nothing can be greater that ULONG_MAX? >> > > Many thanks for your advice. This looks good but it seems this results in different > behavior: When (memlock == LONG_MAX) && !capable(CAP_IPC_LOCK), we would fail now > while it will always success without this change. We should avoid this difference. > Or am I miss something? Maybe the origin patch is more suitable and > simple? Interesting. I think that is an unintended and necessary bug fix. When memlock == LONG_MAX that means inc_rlimit_ucounts failed. It either failed because at another level the limit was exceeded or because the counter wrapped. In either case it is not appropriate to succeed if inc_rlimit_ucounts detects a failure. Which is a long way of saying I think we really want the simplification because it found and fixed another bug as well. Without the simplification I don't think I will be confident the code is correct. Eric > Thanks. > >> Something like this? >> >> diff --git a/mm/mlock.c b/mm/mlock.c >> index 8f584eddd305..e7eabf5193ab 100644 >> --- a/mm/mlock.c >> +++ b/mm/mlock.c >> @@ -827,13 +827,12 @@ int user_shm_lock(size_t size, struct ucounts *ucounts) >> >> locked = (size + PAGE_SIZE - 1) >> PAGE_SHIFT; >> lock_limit = rlimit(RLIMIT_MEMLOCK); >> - if (lock_limit == RLIM_INFINITY) >> - allowed = 1; >> - lock_limit >>= PAGE_SHIFT; >> + if (lock_limit != RLIM_INFINITY) >> + lock_limit >>= PAGE_SHIFT; >> spin_lock(&shmlock_user_lock); >> memlock = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked); >> >> - if (!allowed && (memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) { >> + if ((memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) { >> dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked); >> goto out; >> } >> >>> >>> Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts") >>> Signed-off-by: Miaohe Lin >>> Acked-by: Hugh Dickins >>> --- >>> v1->v2: >>> correct Fixes tag and collect Acked-by tag >>> Thanks Hugh for review! >>> --- >>> mm/mlock.c | 1 + >>> 1 file changed, 1 insertion(+) >>> >>> diff --git a/mm/mlock.c b/mm/mlock.c >>> index 29372c0eebe5..efd2dd2943de 100644 >>> --- a/mm/mlock.c >>> +++ b/mm/mlock.c >>> @@ -733,6 +733,7 @@ int user_shm_lock(size_t size, struct ucounts *ucounts) >>> } >>> if (!get_ucounts(ucounts)) { >>> dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked); >>> + allowed = 0; >>> goto out; >>> } >>> allowed = 1; >> >> Eric >> . >>