From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AE94D381AFF; Sat, 9 May 2026 03:28:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778297326; cv=none; b=hBwMiIuJKoqefGu9dE/0NFPmSaTIpbfAkJun6tRFA5O+zIwIkEB7/46aI5JHIDShiQ5nlLIg2hyW8e8v9W4Pjj3sWDbVVNsEjHKH4PeUOvc3pY3WEIL/cM2HWgM8Ae9suVz75z5oSDvdgN3bq3/BE08A2CJhUvr1HFBCgg5XOmM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778297326; c=relaxed/simple; bh=imYWbFyLism7kfVJrqxg5opLuNGgNjD7c1knO+Nknas=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition; b=hus+3MiKykx5U/QFQGIkG8Sp8DwYOUdOZrEm2W0VBcLUSPZi/kdsNiYpUnIm1TBfxDoXxVinNXJpNnoL+Vu6fF/rSFd+hy9O2GVtC+ylc9rJgK93xyz5pzc0rJ+ZeaF3AiyEkIN1b3mc4qDQIgNWpmvixqHxkeOZbpgqgRlD7iU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Bnp6jaBl; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Bnp6jaBl" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BCA8FC2BCB0; Sat, 9 May 2026 03:28:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778297326; bh=imYWbFyLism7kfVJrqxg5opLuNGgNjD7c1knO+Nknas=; h=Date:From:To:Cc:Subject:From; b=Bnp6jaBl54RKWeLqgkHySc2fw7fNDUP0cpzNvpIeektzSKcXX0kQc1jTqagqlBsUl B+47MwWYhJ9llUwIx1M3G3h9SXrZeA0AWhlenspZWO8T/n+anQWk0oOIP75ARbSBQm tdPZ3/npiL5BfkZk8cVUghRf/wcAsWnr69wH5ZK3t4xp4nCq6ZcEklezaDux4RsdSl ANa/TB0E6ANc8zHn6efehILbwlp4npvYIXrKBH6IbxfYsFu+2enOszjt1m+5pp7iGz Qk+igvkBCUmnWRCddkZUwOH+W1QnDg+cKVVIQdkkOHbGrw0Hvtm7vsOhwPxsBpgYxC dos659gGVs36Q== Date: Fri, 8 May 2026 21:28:43 -0600 From: "Gustavo A. R. Silva" To: Kees Cook Cc: linux-kernel@vger.kernel.org, "Gustavo A. R. Silva" , linux-hardening@vger.kernel.org Subject: [PATCH][next] stddef: Document designated initializer semantics for __TRAILING_OVERLAP() Message-ID: Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Document the designated initializer behavior for overlapping storage between NAME and MEMBERS, and clarify the implications for static initialization to help avoid unintended overwrites. Signed-off-by: Gustavo A. R. Silva --- include/linux/stddef.h | 56 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) diff --git a/include/linux/stddef.h b/include/linux/stddef.h index 80b6bfb944f0..36c91c725546 100644 --- a/include/linux/stddef.h +++ b/include/linux/stddef.h @@ -100,6 +100,62 @@ enum { * Creates a union between a flexible-array member (FAM) in a struct and a set * of additional members that would otherwise follow it. * + * Beware that, as this helper encloses TYPE NAME and MEMBERS in the same + * union, designated initializers for MEMBERS may overwrite portions + * previously initialized through NAME. + * + * For example: + * + * struct flex { + * size_t count; + * u8 fam[]; + * }; + * + * struct composite { + * ... + * __TRAILING_OVERLAP(struct flex, flex, fam, __packed, + * u8 data; + * ); + * } __packed; + * + * static struct composite comp = { + * .flex = { + * .count = 1, + * }, + * .data = 2, + * }; + * + * In the example above, .flex and .data initialize different views of the same + * union storage. Since .data is initialized last, it _may_ overwrite portions + * previously initialized through .flex, leading to .flex.count being zeroed + * out. + * + * A couple of alternatives are show below. + * + * Initialize only one view of the overlapped storage and assign the rest + * at run time: + * + * static struct composite comp = { + * .flex = { + * .count = 1, + * }, + * }; + * + * static void foo(void) + * { + * comp.data = 2; + * ... + * } + * + * (Compiler Explorer test code: https://godbolt.org/z/zz4K1Ejvf) + * + * Alternatively, move the entire initialization to run time. + * + * For an example of stack-based inialization see commit 5e54510a9389 + * ("acpi: nfit: intel: avoid multiple -Wflex-array-member-not-at-end warnings") + * + * Link: https://git.kernel.org/linus/5e54510a9389caa9 + * * @TYPE: Flexible structure type name, including "struct" keyword. * @NAME: Name for a variable to define. * @FAM: The flexible-array member within @TYPE -- 2.51.0