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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 9B34FECAAD3 for ; Thu, 15 Sep 2022 02:58:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:CC:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=RmamC19wEevyIi/1YA21kosHEybrHHMDoIHgkKQx6I0=; b=YRHs/Azo3aamHM UgoOk1Y55+Sp+WDpxYguUHC+x7e+jSyn5x62vJzczCTB3aBOQ3OeHZTyUJ33k6F80WdKcEZ0Lrv/n f754Y/5lt14GZRN3M5ZsUsfY2iqBBw70KctoryB8PuGxl0irHuIqOiIHfxISpLY5Ql3hfA+m627bi 2MNfv0PCGMih+NKQkHhBdZUwkGSNfX51dQfUFuQx2K7pBhjnu7V65cFiEvMy/0ymYQMT3jZONYdtp ZsnpA/uUA1KuWm6RII646aITf4WwNEumClm/r9qqrwe1VLewVvDy8eGyC0obNUIsAYGUQ8npmpPUy DCqKLhTWf2KYtdC4EoZw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1oYf43-00FwyX-Bh; Thu, 15 Sep 2022 02:57:19 +0000 Received: from szxga01-in.huawei.com ([45.249.212.187]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1oYf3z-00Fwod-0i for linux-arm-kernel@lists.infradead.org; Thu, 15 Sep 2022 02:57:17 +0000 Received: from dggpemm500023.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4MShb43Vj9zlVvj; Thu, 15 Sep 2022 10:53:04 +0800 (CST) Received: from dggpemm500013.china.huawei.com (7.185.36.172) by dggpemm500023.china.huawei.com (7.185.36.83) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Thu, 15 Sep 2022 10:57:03 +0800 Received: from [10.67.108.67] (10.67.108.67) by dggpemm500013.china.huawei.com (7.185.36.172) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Thu, 15 Sep 2022 10:57:03 +0800 Message-ID: <6a61aa57-141f-039c-5a2d-b2d79fecb8c2@huawei.com> Date: Thu, 15 Sep 2022 10:56:58 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.0 Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn} Content-Language: en-US To: Michael Matz , Borislav Petkov CC: Josh Poimboeuf , , Peter Zijlstra , Indu Bhagat , Nick Desaulniers , , "Jose E. Marchesi" , Miroslav Benes , "Mark Rutland" , Will Deacon , , , , , "Ard Biesheuvel" , Sathvika Vasireddy , Christophe Leroy , Mark Brown References: <20220909180704.jwwed4zhwvin7uyi@treble> From: Chen Zhongjin In-Reply-To: X-Originating-IP: [10.67.108.67] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To dggpemm500013.china.huawei.com (7.185.36.172) X-CFilter-Loop: Reflected X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220914_195715_439950_14DD52B9 X-CRM114-Status: GOOD ( 41.04 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi, On 2022/9/12 22:17, Michael Matz wrote: > Hey, > > On Mon, 12 Sep 2022, Borislav Petkov wrote: > >> Micha, any opinions on the below are appreciated. >> >> On Fri, Sep 09, 2022 at 11:07:04AM -0700, Josh Poimboeuf wrote: >>> difficult to ensure correctness. Also, due to kernel live patching, the >>> kernel relies on 100% correctness of unwinding metadata, whereas the >>> toolchain treats it as a best effort. > Unwinding certainly is not best effort. It's 100% reliable as far as the > source language or compilation options require. But as it doesn't > touch the discussed features I won't belabor that point. > > I will mention that objtool's existence is based on mistrust, of persons > (not correctly annotating stuff) and of tools (not correctly heeding those > annotations). The mistrust in persons is understandable and can be dealt > with by tools, but the mistrust in tools can't be fixed by making tools > more complicated by emitting even more information; there's no good reason > to assume that one piece of info can be trusted more than other pieces. > So, if you mistrust the tools you have already lost. That's somewhat > philosophical, so I won't beat that horse much more either. > > Now, recovering the CFG. I'll switch order of your two items: > > 2) noreturn function > >>> .pushsection .annotate.noreturn >>> .quad func1 >>> .quad func2 >>> .quad func3 >>> .popsection > This won't work for indirect calls to noreturn functions: > > void (* __attribute__((noreturn)) noretptr)(void); > int callnoret (int i) > { > noretptr(); > return i + 32; > } > > The return statement is unreachable (and removed by GCC). To know that > you would have to mark the call statements, not the individual functions. > All schemes that mark functions that somehow indicates a meaningful > difference in the calling sequence (e.g. the ABI of functions) have the > same problem: it's part of the call expressions type, not of individual > decls. > > Second problem: it's not extensible. Today it's noreturn functions you > want to know, and tomorrow? So, add a flag word per entry, define bit 0 > for now to be NORETURN, and see what comes. Add a header with a version > (and/or identifier) as well and it's properly extensible. For easy > linking and identifying the blobs in the linked result include a length in > the header. If this were in an allocated section it would be a good idea > to refer to the symbols in a PC-relative manner, so as to not result in > runtime relocations. In this case, as it's within a non-alloc section > that doesn't matter. So: > > .section .annotate.functions > .long 1 # version > .long 0xcafe # ident > .long 2f-1f # length > 1: > .quad func1, 1 # noreturn > .quad func2, 1 # noreturn > .quad func3, 32 # something_else_but_not_noreturn > ... > 2: > .long 1b-2b # align and "checksum" > > It might be that the length-and-header scheme is cumbersome if you need to > write those section commands by hand, in which case another scheme might > be preferrable, but it should somehow be self-delimiting. > > For the above problem of indirect calls to noreturns, instead do: > > .text > noretcalllabel: > call noreturn > othercall: > call really_special_thing > .section .annotate.noretcalls > .quad noretcalllabel, 1 # noreturn call > .quad othercall, 32 # call to some special(-ABI?) function > > Same thoughts re extensibility and self-delimitation apply. > > 1) jump tables > >>> Create an .annotate.jump_table section which is an array of the >>> following variable-length structure: >>> >>> struct annotate_jump_table { >>> void *indirect_jmp; >>> long num_targets; >>> void *targets[]; >>> }; > It's very often the case that the compiler already emits what your > .targets[] member would encode, just at some unknown place, length and > encoding. So you would save space if you instead only remember the > encoding and places of those jump tables: We have found some anonymous information on x86 in .rodata. I'm not sure if those are *all* of Josh wanted on x86, however for arm64 we did not found that in the same section so it is a problem on arm64 now. Does the compiler will emit these for all arches? At lease I tried and didn't find anything meaningful (maybe I omitted it). Best, Chen > struct { > void *indirect_jump; > long num_tables; > struct { > unsigned num_entries; > unsigned encoding; > void *start_of_table; > } tables[]; > }; > > The usual encodings are: direct, PC-relative, relative-to-start-of-table. > Usually for a specific jump instruction there's only one table, so > optimizing for that makes sense. For strange unthought-of cases it's > probably a good idea to have your initial scheme as fallback, which could > be indicated by a special .encoding value. > >>> For example, given the following switch statement code: >>> >>> .Lswitch_jmp: >>> // %rax is .Lcase_1 or .Lcase_2 >>> jmp %rax > So, usually %rax would point into a table (somewhere in .rodata/.text) > that looks like so: > > .Ljump_table: > .quad .Lcase_1 - .Ljump_table > .quad .Lcase_2 - .Ljump_table > > (for position-independend code) > > and hence you would emit this as annotation: > > .quad .Lswitch_jmp > .quad 1 # only a single table > .long 2 # with two entries > .long RELATIVE_TO_START # all entries are X - start_of_table > .quad .Ljump_table > > In this case you won't save anything of course, but as soon as there's a > meaningful number of cases you will. > > Again, if that info would be put into an allocated section you would want > to use relative encodings of the addresses to avoid runtime relocs. And > the remarks about self-delimitation and extensibility also apply here. > >>> Alternatives >>> ------------ >>> >>> Another idea which has been floated in the past is for objtool to read >>> DWARF (or .eh_frame) to help it figure out the control flow. That >>> hasn't been tried yet, but would be considerably more difficult and >>> fragile IMO. > While noreturn functions are marked in the debug info, noreturn > function types currently aren't quite correct. And jump-tables aren't > marked at all, so that would lose. > > > Ciao, > Michael. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel