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=-2.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT 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 A661AC43381 for ; Thu, 28 Mar 2019 14:40:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 75F49217F9 for ; Thu, 28 Mar 2019 14:40:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="OoQhSR+i" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726367AbfC1OkC (ORCPT ); Thu, 28 Mar 2019 10:40:02 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:42688 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725849AbfC1OkB (ORCPT ); Thu, 28 Mar 2019 10:40:01 -0400 Received: by mail-pg1-f194.google.com with SMTP id p6so11708616pgh.9 for ; Thu, 28 Mar 2019 07:40:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=su5hUDVGA5wTPt+GZUHCzxW06LCBzo43uhfJs0GTLis=; b=OoQhSR+iKkIWNr/jpeg6QvhAN51oCNMtZeMAz9aSGP40ppc0YnS4WUELW3lhUwjUeZ ndcoFzMd5GGap1GCgZ8SwCddCu2saRDJ6ok2+/ZGJHRNgpkliRHHJfgnZ+ahNvrjs5KW 9WX+Vv5N3ASKe4fayeObXHa/jkv9B8JymoGL8= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=su5hUDVGA5wTPt+GZUHCzxW06LCBzo43uhfJs0GTLis=; b=lLOlNdwqY8bVY0BE/yIAprcnUMOQiYClduZkDza9fXGsBFCelZXzVUs47wBuRhNHCM 1ducWsIRAMRaUnXGZDYWkY6tzlvIqCKSHl6dG1vHAHs8UMJlPS0sK/M/Jp4TntxXGFH9 hhfPdUX1JqcPx5O+CFTqqW+oDCLkfq7GO8cAMVU0P8W+F4an3lpBERzjpWr0IjHekkn+ vRFF5nUrkoa1C0529+CO/MpiqUgJ6Sym4HiGE6qzQJ4ELG52GBajLgVXhhSCXqBx8snj 2cBfMAwK5zHbsINPKVu5qgBu/8yHVP/RVcJTrFg3+j9lCSKYm2MzNvF5trC/u+tb/AmB MMbw== X-Gm-Message-State: APjAAAVwVO/upor/mnPzccbNTR2GfQ3nn4EH9Es5g52KjV+jLyikV1+D MHjAnihD9ftiBtbMjirc1CTxJA== X-Google-Smtp-Source: APXvYqw3tQ+i3uL9g0NHp+mIBhnfkpkTFqdE9c/sRudDrMvFebnVAq1yOdlvIl0bYClQHeCZeKFm5Q== X-Received: by 2002:a65:6383:: with SMTP id h3mr39458734pgv.11.1553784000843; Thu, 28 Mar 2019 07:40:00 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id u13sm49809367pfa.169.2019.03.28.07.39.59 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 28 Mar 2019 07:40:00 -0700 (PDT) Date: Thu, 28 Mar 2019 10:39:58 -0400 From: Joel Fernandes To: Oleg Nesterov Cc: Jann Horn , Kees Cook , "Eric W. Biederman" , LKML , Android Kernel Team , Kernel Hardening , Andrew Morton , Matthew Wilcox , Michal Hocko , "Reshetova, Elena" Subject: Re: [PATCH] Convert struct pid count to refcount_t Message-ID: <20190328143958.GB261521@google.com> References: <20190327145331.215360-1-joel@joelfernandes.org> <20190328023432.GA93275@google.com> <20190328142619.GA19441@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190328142619.GA19441@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 28, 2019 at 03:26:19PM +0100, Oleg Nesterov wrote: > On 03/27, Joel Fernandes wrote: > > > > Also, based on Kees comment, I think it appears to me that get_pid and > > put_pid can race in this way in the original code right? > > > > get_pid put_pid > > > > atomic_dec_and_test returns 1 > > atomic_inc > > kfree > > > > deref pid /* boom */ > > ------------------------------------------------- > > > > I think get_pid needs to call atomic_inc_not_zero() > > No. > > get_pid() should only be used if you already have a reference or you do > something like > > rcu_read_lock(); > pid = find_vpid(); > get_pid(); > rcu_read_lock(); > > in this case we rely on call_rcu(delayed_put_pid) which drops the initial > reference. > > If put_pid() sees pid->count == 1, then a) nobody else has a reference and > b) nobody else can find this pid on rcu-protected lists, so it is safe to > free it. I agree. Check my reply to Jann, I already replied to him about this. thanks!