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=-5.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 54CC4C282DD for ; Mon, 8 Apr 2019 00:36:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 18A3C20857 for ; Mon, 8 Apr 2019 00:36:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="bP2uTrkh" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726501AbfDHAgu (ORCPT ); Sun, 7 Apr 2019 20:36:50 -0400 Received: from mail-pl1-f196.google.com ([209.85.214.196]:44667 "EHLO mail-pl1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726326AbfDHAgt (ORCPT ); Sun, 7 Apr 2019 20:36:49 -0400 Received: by mail-pl1-f196.google.com with SMTP id g12so6258141pll.11 for ; Sun, 07 Apr 2019 17:36:48 -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=mUF7FisCWPJBcyY+1uY0DR4+j9qrIcKSbRWtjK3YtW8=; b=bP2uTrkh739fzSEIqWmyX/sG6NKbgLlBEcO5pUqvbm5z+aN6K3VaMPnW/2QA9Wz44b l3N6uPAeWeFSP1TMOIGsmwa8qjrs6+GJA8hSsrvshaJ6UsZYJuQ2QpJSMj2vGcJ9Uoay s/srx7huOPqF0Za9+Wb0oLU1a9Yrn7RSEZ9pY= 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=mUF7FisCWPJBcyY+1uY0DR4+j9qrIcKSbRWtjK3YtW8=; b=iGJvsHfz/X05TBFlBIjePVyAU+AX06CyMytN4pknAih6a8S3Z3shk3fkCBnD1EOnR5 zrxKrraA3X9AXSxZOLib6/QeZFcSeCFy9gyXLbKOTCHtHopuerBsTn8vogZ5+dq+U1N9 SLf6Ab3SjwbbZrUhHwq++z7a9s3J+jBe9JuKHE0NEGirTAy0gZzJR3K1789DjXgwcf2e fjRHGN+upBUpbcXOI0mtJkjJ7GUGi0AjuP8kgSSPKPmHoAza0CkEY/aWg/zAnn9xgvza I+iZkXDKHfbcp+bOUUfsfH+LtSgvYiSpk6uCH+NP86SCXCZidK+YNOiUeOa4G80RwejD H52Q== X-Gm-Message-State: APjAAAV7F4ogWHmoGe0TlrmHUzqBuLsJHGCs3r8ChimrXPX1NfAqD9m+ r8nCo4mYMoInxw2z1xHLTyeIlA== X-Google-Smtp-Source: APXvYqyEcxluh0LRXUByCtbfcaG1gVCzjI9vRF+orWx6aBtjmYnzxp/Thsz0QHzd2rPbi2TCPhYnXA== X-Received: by 2002:a17:902:b7cc:: with SMTP id v12mr19536417plz.166.1554683808261; Sun, 07 Apr 2019 17:36:48 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id 6sm5669455pft.64.2019.04.07.17.36.46 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 07 Apr 2019 17:36:47 -0700 (PDT) Date: Sun, 7 Apr 2019 20:36:46 -0400 From: Joel Fernandes To: "Paul E. McKenney" , Jessica Yu Cc: Mathieu Desnoyers , rcu , linux-kernel , Ingo Molnar , Lai Jiangshan , dipankar , Andrew Morton , Josh Triplett , Thomas Gleixner , Peter Zijlstra , rostedt , David Howells , Eric Dumazet , fweisbec , Oleg Nesterov , linux-nvdimm , dri-devel , amd-gfx Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules Message-ID: <20190408003646.GA135726@google.com> References: <161156422.1435.1554219247444.JavaMail.zimbra@efficios.com> <20190403133243.GE4102@linux.ibm.com> <1028306587.504.1554301662374.JavaMail.zimbra@efficios.com> <20190403162039.GA14111@linux.ibm.com> <20190405232835.GA24702@linux.ibm.com> <20190406230613.GA187766@google.com> <20190407133941.GC14111@linux.ibm.com> <20190407135937.GA30053@linux.ibm.com> <20190407154613.GA2770@localhost> <20190407170514.GE14111@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190407170514.GE14111@linux.ibm.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 Sun, Apr 07, 2019 at 10:05:14AM -0700, Paul E. McKenney wrote: > On Sun, Apr 07, 2019 at 03:46:13PM +0000, Joel Fernandes wrote: > > On Sun, Apr 07, 2019 at 06:59:37AM -0700, Paul E. McKenney wrote: > > > On Sun, Apr 07, 2019 at 06:39:41AM -0700, Paul E. McKenney wrote: > > > > On Sat, Apr 06, 2019 at 07:06:13PM -0400, Joel Fernandes wrote: > > > > > > [ . . . ] > > > > > > > > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > > > > > > index f8f6f04c4453..c2d919a1566e 100644 > > > > > > --- a/include/asm-generic/vmlinux.lds.h > > > > > > +++ b/include/asm-generic/vmlinux.lds.h > > > > > > @@ -338,6 +338,10 @@ > > > > > > KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \ > > > > > > __stop___tracepoints_ptrs = .; \ > > > > > > *(__tracepoints_strings)/* Tracepoints: strings */ \ > > > > > > + . = ALIGN(8); \ > > > > > > + __start___srcu_struct = .; \ > > > > > > + *(___srcu_struct_ptrs) \ > > > > > > + __end___srcu_struct = .; \ > > > > > > } \ > > > > > > > > > > This vmlinux linker modification is not needed. I tested without it and srcu > > > > > torture works fine with rcutorture built as a module. Putting further prints > > > > > in kernel/module.c verified that the kernel is able to find the srcu structs > > > > > just fine. You could squash the below patch into this one or apply it on top > > > > > of the dev branch. > > > > > > > > Good point, given that otherwise FORTRAN named common blocks would not > > > > work. > > > > > > > > But isn't one advantage of leaving that stuff in the RO_DATA_SECTION() > > > > macro that it can be mapped read-only? Or am I suffering from excessive > > > > optimism? > > > > > > And to answer the other question, in the case where I am suffering from > > > excessive optimism, it should be a separate commit. Please see below > > > for the updated original commit thus far. > > > > Actually the vmlinux.lds.h file is unused for module building. For ex, if you > > delete include/asm-generic/vmlinux.lds.h , then you can still build > > rcutorture.ko. Did I miss something obvious? In that case the vmlinux.lds.h > > are not needed since the __section annotations automatically place the srcu > > structs in a separate section. > > Hard to argue given that I just deleted include/asm-generic/vmlinux.lds.h, > touched kernel/rcu/rcutorture.c, and rebuilt the corresponding .ko > without errors. ;-) > > Hmmm... Is there some way to place a section into a read-only page, > for example, tagged onto the text section for that module? That would > get rid of a class of bugs, if nothing else. Strictly speaking, the array of pointers in the new srcu section are fixed up at runtime because the srcu_struct(s) they point to can be loaded at a dynamic location in memory. The srcu_struct(s) are themselves in the .bss section of the module and their locations depend on where the .bss section of the module is loaded in memory at load time. I agree that after such relocation fixups are done, there is no reason to keep the array-of-pointers section readable but unfortunately I couldn't figure a way out to make them read-only post the relocations. I copied Jessica Yu who maintains module loading for any input. Jessica, as a summary, we are trying to create a custom ELF section of srcu_struct pointers in kernels modules, and then make the module loader do SRCU initialization from structs pointed to by this section. The srcu_struct themselves are defined on the .bss section. Is there any way we can make this pointer array section read-only *after* the relocation fixups of the array are completed? > > Let me know if you would like me to send a patch separately, or if the > > appended patch for the same in my previous email suffices. > > Please do resend as a formal patch with the above in the commit log. > I doubt that I am the only one needing a bit of module-build education! > And thank you for providing that education, by the way! Sounds great, I will go ahead and send out a patch in the morning for this part. > > > And may I have your Tested-by? > > > > Absolutely, please do and thanks! > > Done, and thank you for giving it a go! You're very welcome. thanks, - Joel