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 80D0632F759; Sat, 28 Feb 2026 08:55:01 +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=1772268906; cv=none; b=l9VzPMV/Vkn0xhK+5xJMwX5xikJWI63uxlBX6uJPIBCf/VyqnrOAjY+EqcHKkdOKoGLnutiQwmNTCO13Q4DHWfMl59ORkWns87Dtq6VgH8YlRSHDCnjkOk2C4VEoOTfxbAc4tTl9yzG2bbkiq7FOI0djzmsuXsfjkU3R4aUILXs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772268906; c=relaxed/simple; bh=aelZWqczn7JCAwwAEKXsndipwav0OcO3KOHyn66YLdk=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Buzw8dH6YXQpjd6M6y9Rhuj2hNJfCvTFd+swvD/VCWP7ror6XdG+kmYH1EP9crJufKrKp1G+85c4x70QS3HwBLwoH+kjxj9IXYEj3yTGix2IdwViIL8sWqSScqovXGtI8vxQFDoH4wwbxNJ4Bk7S5Qn/HmpfC0P9X6zz9UhWv1M= 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=lJtyr880; 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="lJtyr880" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=3bujMRIIY9It6R9k+seKYahrmKxSHWOc1fd7A1apitw=; b=lJtyr880lZV9CUvG01HDzjgg5O GnyCH0Xy6052iGENnFGULQkE9JCNksjXcEWA0P2iM0RD1dv/fiYrkIcpT49rLAJHDQyE8i0TWwTNG TTc4uAxq6jeE5R5g45m/d8PlvgYwYbBW6oZeIYc8Zn9aaEEtoQDaphwtjrVlXcsWWzF2zdpekuQ7n K9mmS4K9etoUSHbmqlPaqPLZNJ4VZtTx6HgIbfF9arnjDm6HQxMgHD7L6Hcr0xUFrJS1/h2N8lbSC fRsCskBLuRkay4H+cEqQ/BQGrrGbGHZ8Hs21m/gQAeACRPBVFhIery5oLN/zNw3fSrYxvEo0y3IBV rDaXsRzA==; Received: from [172.31.31.148] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vwG6C-0000000ERK0-1iXz; Sat, 28 Feb 2026 08:54:57 +0000 Message-ID: Subject: Re: [PATCH] KVM: x86: Fix C++ user API for structures with variable length arrays From: David Woodhouse To: Kees Cook Cc: "Gustavo A. R. Silva" , daniel@iogearbox.net, gustavoars@kernel.org, jgg@ziepe.ca, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Sean Christopherson , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" Date: Sat, 28 Feb 2026 08:54:56 +0000 In-Reply-To: <948BD627-E51D-4959-9745-3AB390F04D0D@kernel.org> References: <97d40dd0e6abaf28f43d4d8ccf9c547a16c52e33.camel@infradead.org> <202602261053.78753BF1C@keescook> <718cbe24ffb546b97db8af94521da067d7437d70.camel@infradead.org> <948BD627-E51D-4959-9745-3AB390F04D0D@kernel.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-juAUHL0AilHHmRSZIdlK" 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 --=-juAUHL0AilHHmRSZIdlK Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2026-02-27 at 16:43 -0800, Kees Cook wrote: >=20 >=20 > On February 27, 2026 12:29:11 AM PST, David Woodhouse wrote: > > On Thu, 2026-02-26 at 11:02 -0800, Kees Cook wrote: > > >=20 > > > > Also put the header fields into a struct_group() to provide (in C) = a > > > > separate struct (e.g 'struct kvm_msrs_hdr') without the trailing VL= A. > > >=20 > > > Right, my only worry is if C++ would want those header structs too. I= n > > > that case, you'd probably want to use a macro to include them (since = not > > > all compilers are supporting transparent struct members yet): > > >=20 > > > #define __kvm_msrs_hdr \ > > > __u32 nmsrs; /* number of msrs in entries */ \ > > > __u32 pad > > >=20 > > > struct kvm_msrs_hdr { > > > __kvm_msrs_hdr; > > > }; > > >=20 > > > struct kvm_msrs { > > > __kvm_msrs_hdr; > > > __DECLARE_FLEX_ARRAY(struct kvm_msr_entry, entries); > > > }; > >=20 > > Hm, does the struct_group() not also depend on the compiler supporting > > anonymous struct members? Is there a distinction I'm missing? >=20 > Sorry, I should have been mor clear: the _tag_ that struct_group > creates isn't supported for all C++ compilers (so is commented out > for C++) so if a C++ project wanted to use the struct by its tag > ("struct kvm_msrs_hdr"), you'd need the construct I wrote above. >=20 Understood, yes. I was actually getting confused about why we'd use that #define __kvm_msrs_hdr, and why we wouldn't do it like this: struct kvm_msr_hdr { __u32 nmsrs; __u32 pad; } struct kvm_msrs { struct kvm_msr_hdr /* anonymous */; __DECLARE_FLEX_ARRAY(struct kvm_msr_entry, entries); }; (In fact, that doesn't even work with GCC). Nevertheless, my question at the time was intended as: if we can't do it like *that*, then surely the struct_group() construct will fail on those compilers too? But that was a brain fart because what struct_group() does is actually more like: struct kvm_msrs { union { struct { __u32 nmsrs; __u32 pad; }; struct /* foo in C */ { __u32 nmsrs; __u32 pad; } foo; }; __DECLARE_FLEX_ARRAY(struct kvm_msr_entry, entries); }; And that *does* work. But yeah, I think C++ should be fine without the new header structs. What I posted at least allows it to *build* with actual instantiations of these structures with real payload again. --=-juAUHL0AilHHmRSZIdlK Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 aDJAwtycb8yXtILj+TvfhLFhafxroXrflspavejQkEiHjNjtHnwbZ+o43g0/yxjwnarGI3kgcak7 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