From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 6FB91279DAF; Mon, 23 Feb 2026 14:28:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771856897; cv=none; b=KxzY29lM/Lf5aKdebLwCQB6G8AflG/vbLmdNrHn4mWoj+MW1kAXxAg0wVB6WXtVgYtlVF03mYsM1PIJGmXkMAjB6cb4eUKzTnexYQvge9hyvl1rYv/i3JqaYeNnit47SYdzJz2e5eLPlPLfhJO6JAzh64YU//Ji3KMQodwQY7DY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771856897; c=relaxed/simple; bh=liGe/f8nTjXBTYmwSDWmgoO41MKea1Njvkigd/rRY9U=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:Content-Type: MIME-Version; b=Ghj/zMs+KmdDHzPrmtieKsJTaU0wHYE7cPKtiXqXJ32U4DVkY60EoqhMJA+5dIRCnRQ9ugQx9pkAYKOlz99+T8WwitZ/SKbCLw4nbeop3zYc6LrNCH5RmyCsXL0WJIHao+py3naCV3N9M/A/NgXA5orzUVYwalTl9ZD1y6zv2/M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=desiato.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=UEFKXPdm; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=desiato.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="UEFKXPdm" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=MIME-Version:Content-Type:In-Reply-To: Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:References; bh=EZHhG/Zc1gLhsN5qSAFjSEGcmOpsNH01QGTcHKmriSE=; b=UEFKXPdmoqLCSw6xExCt3Hg+DW DYUrOg0pjAmg5OgNsxCpBKU1XRXS/+Bfm3KMCZENVCUrQbu+ZRmwDvm45KiJOlzvrd632J4xa4WOr znJpismFbRHdX8m5btzjZZSEWrgfdSzcAxz8bzuq92TNlbC+B3g2lYb+GZjZA7KrD1sYDVZ3mSCWv hYcI/IxxAc7A6E+Dsq34nPGvu94ZDcV0vtGq18q71nz17aNC/XWrwVf6eMitYtZ8qGp2NBa8np4cA J7Q9OZEyP7VNPfxNNhKl4B8x/2nH8ofzTO2iPk0nmialc/q22Wpix9rSmKW8ocxzKIzveleq+jy0/ Ln6+RgXA==; Received: from [172.31.31.148] (helo=u09cd745991455d.lumleys.internal) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vuWux-00000005F94-1Acj; Mon, 23 Feb 2026 14:28:11 +0000 Message-ID: Subject: Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexible-array members From: David Woodhouse To: keescook@chromium.org Cc: daniel@iogearbox.net, gustavoars@kernel.org, jgg@ziepe.ca, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Date: Mon, 23 Feb 2026 14:28:11 +0000 In-Reply-To: <202206281009.4332AA33@keescook> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-qhigXTJDzzvqKXCmlK/G" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html --=-qhigXTJDzzvqKXCmlK/G Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On 2022-07-28 at 10:54:58 -0700, Kees Cook wrote: > The issue here seems to be a collision between "unknown array size" > and known sizes: >=20 > struct bpf_lpm_trie_key { > __u32 prefixlen; /* up to 32 for AF_INET, 128 for AF_INET6= */ > __u8 data[0]; /* Arbitrary size */ > }; >=20 > struct lpm_key { > struct bpf_lpm_trie_key trie_key; > __u32 data; > }; >=20 > This is treating trie_key as a header, which it's not: it's a complete > structure. :) >=20 > Perhaps: >=20 > struct lpm_key { > __u32 prefixlen; > __u32 data; > }; >=20 > I don't see anything else trying to include bpf_lpm_trie_key. What was the eventual conclusion here? These changes broke userspace compilation, because a *lot* of these structures ending with VLAs are very much used as headers. QEMU does it, for example... uint64_t kvm_arch_get_supported_msr_feature(KVMState *s, uint32_t index) { struct { struct kvm_msrs info; struct kvm_msr_entry entries[1]; } msr_data =3D {}; ... which works in C but not in C++, which gives an error I can't work out how to avoid. Am I missing something, or did we break our userspace headers for C++ and make them C-only? $ cat test_kvm.cpp=20 #include #include struct msr_data { struct kvm_msrs info; struct kvm_msr_entry entries[1]; }; struct msr_data *alloc_msr_data(void) { return static_cast(malloc(sizeof(struct msr_data))); } $ g++ -c -o test_kvm.o test_kvm.cpp In file included from test_kvm.cpp:2: /usr/include/x86_64-linux-gnu/asm/kvm.h:194:30: error: flexible array membe= r =E2=80=98kvm_msrs::entries=E2=80=99 not at end of =E2=80=98struct msr_dat= a=E2=80=99 194 | struct kvm_msr_entry entries[]; | ^~~~~~~ test_kvm.cpp:6:30: note: next member =E2=80=98kvm_msr_entry msr_data::entri= es [1]=E2=80=99 declared here 6 | struct kvm_msr_entry entries[1]; | ^~~~~~~ test_kvm.cpp:4:8: note: in the definition of =E2=80=98struct msr_data=E2=80= =99 4 | struct msr_data { | ^~~~~~~~ --=-qhigXTJDzzvqKXCmlK/G Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" 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