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=-8.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 0C3BDC4338F for ; Wed, 18 Aug 2021 22:35:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E365B6108D for ; Wed, 18 Aug 2021 22:35:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234550AbhHRWgC (ORCPT ); Wed, 18 Aug 2021 18:36:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50392 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234529AbhHRWgA (ORCPT ); Wed, 18 Aug 2021 18:36:00 -0400 Received: from mail-pg1-x530.google.com (mail-pg1-x530.google.com [IPv6:2607:f8b0:4864:20::530]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 327C6C06179A for ; Wed, 18 Aug 2021 15:35:25 -0700 (PDT) Received: by mail-pg1-x530.google.com with SMTP id 17so3875347pgp.4 for ; Wed, 18 Aug 2021 15:35:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=P+5mSEtJrjnggVNveih68RddTImV6HnC03hWsATd0wA=; b=kRIj8JIqpPRaKtf6CIkL46zXyN+TG7NSk6bw8gu7+8srWnLtPpMsCTfSAl6mGxQkPO AO2tJAjBgrQqg48z/R0w1gBkmv7qgCaYMxOgKctRX1l4WVxZaQ6vNnvX5LAGRKfFudjG PsQYY9oWNpYKfElRnHpaNeEhFchqOLvvFftCGuTzj/AyqbjlyvdzlVVTJ1IB4Ngv7/Ms u4r7kwHlTNVtr67hCv3yPPJX5ARYvjGYympkKth1dgkHgrQuh2AbLYKgiGUMqV8NtvDX +pRCiBA2g2BclAPoPSaP8V7pu3JHnBNUDDtuik7qEFxuxYTic4rAadkAFa3uqJjCsduq ulwQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=P+5mSEtJrjnggVNveih68RddTImV6HnC03hWsATd0wA=; b=nGcYaw3lStL+DgkQ4dqWGtJX9jj1b12fKrSomtAZQY3NM4BQlId1B+mLhVxj7WUY/x hEf7qxRsMTdwinWVIQipma3WgnCT9Ni5LXo+FsniPZTAZVd6veiRUSFYaJ8SlIl67ido BKd0z8VqxKj+fp1887bXFDurbJFLy4N1wZp4vzlJblEs75qdyY07qvDC8fAzbP8oI0i+ 82geUMGmzaVPhMilHMCl5JF8h7w98ehzjhp6Oj+UZciTWDy6ueJLjO3zobchBBBVZSHR GzZ0tkSDon3ivRnmOBG34lA3qhQQgTzMfbuZCqz3bJ5Eh6gVgFEgbuzXebEIfNN4xcJL JdUg== X-Gm-Message-State: AOAM533r2m8yYbv7osunFgMyYvfbIdapouESzPrp5YvJD/ZItice1p6h 0Tyw7m/LQ206RFUD2nBJERwl2ToqEiEbE6MvKluIgA== X-Google-Smtp-Source: ABdhPJwyt02ESgNp2hR1UmZAbrXcfqwpxlM4lrynFVwd2ax1rLD8IK0a6F+uEetO+Gksr5LJ8GXEb6TCnHWEdnZ/Nzg= X-Received: by 2002:a63:311:: with SMTP id 17mr10873581pgd.450.1629326124479; Wed, 18 Aug 2021 15:35:24 -0700 (PDT) MIME-Version: 1.0 References: <20210818060533.3569517-1-keescook@chromium.org> <20210818060533.3569517-6-keescook@chromium.org> In-Reply-To: <20210818060533.3569517-6-keescook@chromium.org> From: Dan Williams Date: Wed, 18 Aug 2021 15:35:13 -0700 Message-ID: Subject: Re: [PATCH v2 05/63] stddef: Introduce struct_group() helper macro To: Kees Cook Cc: Linux Kernel Mailing List , Keith Packard , "Gustavo A . R . Silva" , Rasmus Villemoes , Daniel Vetter , Greg Kroah-Hartman , Andrew Morton , Linux Wireless List , Netdev , Maling list - DRI developers , linux-staging@lists.linux.dev, linux-block@vger.kernel.org, Linux Kbuild mailing list , clang-built-linux , linux-hardening@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 17, 2021 at 11:06 PM Kees Cook wrote: > > Kernel code has a regular need to describe groups of members within a > structure usually when they need to be copied or initialized separately > from the rest of the surrounding structure. The generally accepted design > pattern in C is to use a named sub-struct: > > struct foo { > int one; > struct { > int two; > int three, four; > } thing; > int five; > }; > > This would allow for traditional references and sizing: > > memcpy(&dst.thing, &src.thing, sizeof(dst.thing)); > > However, doing this would mean that referencing struct members enclosed > by such named structs would always require including the sub-struct name > in identifiers: > > do_something(dst.thing.three); > > This has tended to be quite inflexible, especially when such groupings > need to be added to established code which causes huge naming churn. > Three workarounds exist in the kernel for this problem, and each have > other negative properties. > > To avoid the naming churn, there is a design pattern of adding macro > aliases for the named struct: > > #define f_three thing.three > > This ends up polluting the global namespace, and makes it difficult to > search for identifiers. > > Another common work-around in kernel code avoids the pollution by avoiding > the named struct entirely, instead identifying the group's boundaries using > either a pair of empty anonymous structs of a pair of zero-element arrays: > > struct foo { > int one; > struct { } start; > int two; > int three, four; > struct { } finish; > int five; > }; > > struct foo { > int one; > int start[0]; > int two; > int three, four; > int finish[0]; > int five; > }; > > This allows code to avoid needing to use a sub-struct named for member > references within the surrounding structure, but loses the benefits of > being able to actually use such a struct, making it rather fragile. Using > these requires open-coded calculation of sizes and offsets. The efforts > made to avoid common mistakes include lots of comments, or adding various > BUILD_BUG_ON()s. Such code is left with no way for the compiler to reason > about the boundaries (e.g. the "start" object looks like it's 0 bytes > in length), making bounds checking depend on open-coded calculations: > > if (length > offsetof(struct foo, finish) - > offsetof(struct foo, start)) > return -EINVAL; > memcpy(&dst.start, &src.start, offsetof(struct foo, finish) - > offsetof(struct foo, start)); > > However, the vast majority of places in the kernel that operate on > groups of members do so without any identification of the grouping, > relying either on comments or implicit knowledge of the struct contents, > which is even harder for the compiler to reason about, and results in > even more fragile manual sizing, usually depending on member locations > outside of the region (e.g. to copy "two" and "three", use the start of > "four" to find the size): > > BUILD_BUG_ON((offsetof(struct foo, four) < > offsetof(struct foo, two)) || > (offsetof(struct foo, four) < > offsetof(struct foo, three)); > if (length > offsetof(struct foo, four) - > offsetof(struct foo, two)) > return -EINVAL; > memcpy(&dst.two, &src.two, length); > > In order to have a regular programmatic way to describe a struct > region that can be used for references and sizing, can be examined for > bounds checking, avoids forcing the use of intermediate identifiers, > and avoids polluting the global namespace, introduce the struct_group() > macro. This macro wraps the member declarations to create an anonymous > union of an anonymous struct (no intermediate name) and a named struct > (for references and sizing): > > struct foo { > int one; > struct_group(thing, > int two; > int three, four; > ); > int five; > }; > > if (length > sizeof(src.thing)) > return -EINVAL; > memcpy(&dst.thing, &src.thing, length); > do_something(dst.three); > > There are some rare cases where the resulting struct_group() needs > attributes added, so struct_group_attr() is also introduced to allow > for specifying struct attributes (e.g. __align(x) or __packed). > Additionally, there are places where such declarations would like to > have the struct be typed, so struct_group_typed() is added. > > Given there is a need for a handful of UAPI uses too, the underlying > __struct_group() macro has been defined in UAPI so it can be used there > too. > > Co-developed-by: Keith Packard > Signed-off-by: Keith Packard > Signed-off-by: Kees Cook > Acked-by: Gustavo A. R. Silva > Link: https://lore.kernel.org/lkml/20210728023217.GC35706@embeddedor > Enhanced-by: Rasmus Villemoes > Link: https://lore.kernel.org/lkml/41183a98-bdb9-4ad6-7eab-5a7292a6df84@rasmusvillemoes.dk > Enhanced-by: Dan Williams Acked-by: Dan Williams