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 151DE374752; Mon, 23 Feb 2026 19:57:13 +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=1771876638; cv=none; b=J8bYnDWs8ggaFKpF9xBF9BdLGhDpgHNf1aa05J9p+nqnSaUXH9fGGcjKAyjMvgdNzwUXLffTtxdEjkQ2DiGfQF824cxFHPpRYmtwVAiHjqJyPsBk5X+0xqxni2VsZIfXzIWHJ774bG9cpq6tjb1VsEgpcYOgWZuGNQhKM56INAs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771876638; c=relaxed/simple; bh=ORI0gMt/SXGL0wgLseWArXtDjyAoZjIdiK2tEAwAEHY=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=pv42ccauzwEg+aba0gWsp7LtAEazW/Jza628k6J19i+XfxmF5ApCqpUULqwPzNiKvw0BNlrSBqWFo+tdlLakqXOfWu/y7xawUt7HWgou9xI2kkIG+cUoxGqYJCEEGDU8AruuN9n1YhISix5/EH9gMwt3XSnJFw7NSXaIs6C7XVc= 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=TUb2D2+n; 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="TUb2D2+n" 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=tBjq5Z6xw9zNuSi9X1dAM5DorJrHaKhpMl8iroRYu40=; b=TUb2D2+neNO3xtUer0spB7JorQ sfgddZaPGPzu1gG10eAKqH85NfQQolnuBT0e/Yr4chxysPjFaoAFmf0rcn3Zq4jas59ZW5o5WBf06 yhVMs9ne2lDpVuUGL1T8acJMyV0Dn3TAPYvkWpt8PjJXvUdbVwBmb+aV0IukBGzUHWYeHuUr1dVmd 5ve7NmMeytUDGewRTW88XS/PmHTJLWOQ7AS6K7FP9LO/7RQuSvPsbOw8dXaXkJeYoM+y/5BhEcpZh RcXjqVkZMkoO5D98kWvanOlgM0AxaccEMvWUj5soJxbKddc5wGdNqYHXHFu5tY8bOlvU3N7UUTi0N dlUin2Vw==; 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 1vuc3K-00000005q6U-1QfM; Mon, 23 Feb 2026 19:57:10 +0000 Message-ID: Subject: Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexible-array members From: David Woodhouse To: "Gustavo A. R. Silva" , 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 19:57:10 +0000 In-Reply-To: References: Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-a69pBfocK4srBbC7syIY" 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 --=-a69pBfocK4srBbC7syIY Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2026-02-23 at 12:38 +0900, Gustavo A. R. Silva wrote: > > These changes broke userspace compilation, because a *lot* of these > > structures ending with VLAs are very much used as headers. > >=20 > > QEMU does it, for example... > >=20 > > uint64_t kvm_arch_get_supported_msr_feature(KVMState *s, uint32_t index= ) > > { > > =C2=A0=C2=A0=C2=A0=C2=A0 struct { > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct kvm_msrs info; > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct kvm_msr_entry e= ntries[1]; > > =C2=A0=C2=A0=C2=A0=C2=A0 } msr_data =3D {}; > >=20 > >=20 > > ... which works in C but not in C++, which gives an error I can't work > > out how to avoid. > >=20 > > Am I missing something, or did we break our userspace headers for C++ > > and make them C-only? > >=20 > > =C2=A0 $ cat test_kvm.cpp > > #include > > #include > >=20 > > struct msr_data { > > struct kvm_msrs info; > > struct kvm_msr_entry entries[1]; > > }; >=20 > Something like this should eventually land: >=20 > diff --git a/tools/arch/x86/include/uapi/asm/kvm.h b/tools/arch/x86/inclu= de/uapi/asm/kvm.h > index 7ceff6583652..295b2a54b25d 100644 > --- a/tools/arch/x86/include/uapi/asm/kvm.h > +++ b/tools/arch/x86/include/uapi/asm/kvm.h > @@ -192,11 +192,14 @@ struct kvm_msr_entry { > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 __u64 data; > =C2=A0 }; >=20 > -/* for KVM_GET_MSRS and KVM_SET_MSRS */ > -struct kvm_msrs { > +struct kvm_msrs_hdr { > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 __u32 nmsrs; /* number o= f msrs in entries */ > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 __u32 pad; > +}; >=20 > +/* for KVM_GET_MSRS and KVM_SET_MSRS */ > +struct kvm_msrs { > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct kvm_msrs_hdr; > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct kvm_msr_entry ent= ries[]; > =C2=A0 }; I guess that would work, although it's still breaking the userspace API. I was thinking of something along the lines of #ifdef __cplusplus #define __KERNEL_VLA 0 #else #define __KERNEL_VLA #endif struct kvm_msrs { __u32 nmsrs; /* number of msrs in entries */ __u32 pad; struct kvm_msr_entry entries[__KERNEL_VLA]; }; I suppose it could depend on __KERNEL__ instead of __cplusplus too? --=-a69pBfocK4srBbC7syIY 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 nnI9/8Lqpq79tLHYwLajotwLiGTB71AGN5xK+tzB+D4eN9lXayrjcszgbOv2ZCgzExQUAIt98mre 8EggKs9mwtEuKAhYBIP/0K6WsoMnQCcCAwEAAaOCAVwwggFYMBIGA1UdEwEB/wQIMAYBAf8CAQAw HQYDVR0OBBYEFIlICOogTndrhuWByNfhjWSEf/xwMB8GA1UdIwQYMBaAFEXroq/0ksuCMS1Ri6en IZ3zbcgPMA4GA1UdDwEB/wQEAwIBhjAdBgNVHSUEFjAUBggrBgEFBQcDBAYIKwYBBQUHAwIweQYI KwYBBQUHAQEEbTBrMCQGCCsGAQUFBzABhhhodHRwOi8vb2NzcC5kaWdpY2VydC5jb20wQwYIKwYB BQUHMAKGN2h0dHA6Ly9jYWNlcnRzLmRpZ2ljZXJ0LmNvbS9EaWdpQ2VydEFzc3VyZWRJRFJvb3RD QS5jcnQwRQYDVR0fBD4wPDA6oDigNoY0aHR0cDovL2NybDMuZGlnaWNlcnQuY29tL0RpZ2lDZXJ0 QXNzdXJlZElEUm9vdENBLmNybDARBgNVHSAECjAIMAYGBFUdIAAwDQYJKoZIhvcNAQELBQADggEB ACiagCqvNVxOfSd0uYfJMiZsOEBXAKIR/kpqRp2YCfrP4Tz7fJogYN4fxNAw7iy/bPZcvpVCfe/H /CCcp3alXL0I8M/rnEnRlv8ItY4MEF+2T/MkdXI3u1vHy3ua8SxBM8eT9LBQokHZxGUX51cE0kwa uEOZ+PonVIOnMjuLp29kcNOVnzf8DGKiek+cT51FvGRjV6LbaxXOm2P47/aiaXrDD5O0RF5SiPo6 xD1/ClkCETyyEAE5LRJlXtx288R598koyFcwCSXijeVcRvBB1cNOLEbg7RMSw1AGq14fNe2cH1HG W7xyduY/ydQt6gv5r21mDOQ5SaZSWC/ZRfLDuEYwggWbMIIEg6ADAgECAhAH5JEPagNRXYDiRPdl c1vgMA0GCSqGSIb3DQEBCwUAMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYD VQQDExdWZXJva2V5IFNlY3VyZSBFbWFpbCBHMjAeFw0yNDEyMzAwMDAwMDBaFw0yODAxMDQyMzU5 NTlaMB4xHDAaBgNVBAMME2R3bXcyQGluZnJhZGVhZC5vcmcwggIiMA0GCSqGSIb3DQEBAQUAA4IC DwAwggIKAoICAQDali7HveR1thexYXx/W7oMk/3Wpyppl62zJ8+RmTQH4yZeYAS/SRV6zmfXlXaZ sNOE6emg8WXLRS6BA70liot+u0O0oPnIvnx+CsMH0PD4tCKSCsdp+XphIJ2zkC9S7/yHDYnqegqt w4smkqUqf0WX/ggH1Dckh0vHlpoS1OoxqUg+ocU6WCsnuz5q5rzFsHxhD1qGpgFdZEk2/c//ZvUN i12vPWipk8TcJwHw9zoZ/ZrVNybpMCC0THsJ/UEVyuyszPtNYeYZAhOJ41vav1RhZJzYan4a1gU0 kKBPQklcpQEhq48woEu15isvwWh9/+5jjh0L+YNaN0I//nHSp6U9COUG9Z0cvnO8FM6PTqsnSbcc 0j+GchwOHRC7aP2t5v2stVx3KbptaYEzi4MQHxm/0+HQpMEVLLUiizJqS4PWPU6zfQTOMZ9uLQRR ci+c5xhtMEBszlQDOvEQcyEG+hc++fH47K+MmZz21bFNfoBxLP6bjR6xtPXtREF5lLXxp+CJ6KKS blPKeVRg/UtyJHeFKAZXO8Zeco7TZUMVHmK0ZZ1EpnZbnAhKE19Z+FJrQPQrlR0gO3lBzuyPPArV hvWxjlO7S4DmaEhLzarWi/ze7EGwWSuI2eEa/8zU0INUsGI4ywe7vepQz7IqaAovAX0d+f1YjbmC VsAwjhLmveFjNwIDAQABo4IBsDCCAawwHwYDVR0jBBgwFoAUiUgI6iBOd2uG5YHI1+GNZIR//HAw HQYDVR0OBBYEFFxiGptwbOfWOtMk5loHw7uqWUOnMDAGA1UdEQQpMCeBE2R3bXcyQGluZnJhZGVh ZC5vcmeBEGRhdmlkQHdvb2Rob3Uuc2UwFAYDVR0gBA0wCzAJBgdngQwBBQEBMA4GA1UdDwEB/wQE AwIF4DAdBgNVHSUEFjAUBggrBgEFBQcDAgYIKwYBBQUHAwQwewYDVR0fBHQwcjA3oDWgM4YxaHR0 cDovL2NybDMuZGlnaWNlcnQuY29tL1Zlcm9rZXlTZWN1cmVFbWFpbEcyLmNybDA3oDWgM4YxaHR0 cDovL2NybDQuZGlnaWNlcnQuY29tL1Zlcm9rZXlTZWN1cmVFbWFpbEcyLmNybDB2BggrBgEFBQcB AQRqMGgwJAYIKwYBBQUHMAGGGGh0dHA6Ly9vY3NwLmRpZ2ljZXJ0LmNvbTBABggrBgEFBQcwAoY0 aHR0cDovL2NhY2VydHMuZGlnaWNlcnQuY29tL1Zlcm9rZXlTZWN1cmVFbWFpbEcyLmNydDANBgkq hkiG9w0BAQsFAAOCAQEAQXc4FPiPLRnTDvmOABEzkIumojfZAe5SlnuQoeFUfi+LsWCKiB8Uextv iBAvboKhLuN6eG/NC6WOzOCppn4mkQxRkOdLNThwMHW0d19jrZFEKtEG/epZ/hw/DdScTuZ2m7im 8ppItAT6GXD3aPhXkXnJpC/zTs85uNSQR64cEcBFjjoQDuSsTeJ5DAWf8EMyhMuD8pcbqx5kRvyt JPsWBQzv1Dsdv2LDPLNd/JUKhHSgr7nbUr4+aAP2PHTXGcEBh8lTeYea9p4d5k969pe0OHYMV5aL xERqTagmSetuIwolkAuBCzA9vulg8Y49Nz2zrpUGfKGOD0FMqenYxdJHgDCCBZswggSDoAMCAQIC EAfkkQ9qA1FdgOJE92VzW+AwDQYJKoZIhvcNAQELBQAwQTELMAkGA1UEBhMCQVUxEDAOBgNVBAoT B1Zlcm9rZXkxIDAeBgNVBAMTF1Zlcm9rZXkgU2VjdXJlIEVtYWlsIEcyMB4XDTI0MTIzMDAwMDAw MFoXDTI4MDEwNDIzNTk1OVowHjEcMBoGA1UEAwwTZHdtdzJAaW5mcmFkZWFkLm9yZzCCAiIwDQYJ KoZIhvcNAQEBBQADggIPADCCAgoCggIBANqWLse95HW2F7FhfH9bugyT/danKmmXrbMnz5GZNAfj Jl5gBL9JFXrOZ9eVdpmw04Tp6aDxZctFLoEDvSWKi367Q7Sg+ci+fH4KwwfQ8Pi0IpIKx2n5emEg nbOQL1Lv/IcNiep6Cq3DiyaSpSp/RZf+CAfUNySHS8eWmhLU6jGpSD6hxTpYKye7PmrmvMWwfGEP WoamAV1kSTb9z/9m9Q2LXa89aKmTxNwnAfD3Ohn9mtU3JukwILRMewn9QRXK7KzM+01h5hkCE4nj W9q/VGFknNhqfhrWBTSQoE9CSVylASGrjzCgS7XmKy/BaH3/7mOOHQv5g1o3Qj/+cdKnpT0I5Qb1 nRy+c7wUzo9OqydJtxzSP4ZyHA4dELto/a3m/ay1XHcpum1pgTOLgxAfGb/T4dCkwRUstSKLMmpL g9Y9TrN9BM4xn24tBFFyL5znGG0wQGzOVAM68RBzIQb6Fz758fjsr4yZnPbVsU1+gHEs/puNHrG0 9e1EQXmUtfGn4InoopJuU8p5VGD9S3Ikd4UoBlc7xl5yjtNlQxUeYrRlnUSmdlucCEoTX1n4UmtA 9CuVHSA7eUHO7I88CtWG9bGOU7tLgOZoSEvNqtaL/N7sQbBZK4jZ4Rr/zNTQg1SwYjjLB7u96lDP sipoCi8BfR35/ViNuYJWwDCOEua94WM3AgMBAAGjggGwMIIBrDAfBgNVHSMEGDAWgBSJSAjqIE53 a4blgcjX4Y1khH/8cDAdBgNVHQ4EFgQUXGIam3Bs59Y60yTmWgfDu6pZQ6cwMAYDVR0RBCkwJ4ET ZHdtdzJAaW5mcmFkZWFkLm9yZ4EQZGF2aWRAd29vZGhvdS5zZTAUBgNVHSAEDTALMAkGB2eBDAEF AQEwDgYDVR0PAQH/BAQDAgXgMB0GA1UdJQQWMBQGCCsGAQUFBwMCBggrBgEFBQcDBDB7BgNVHR8E dDByMDegNaAzhjFodHRwOi8vY3JsMy5kaWdpY2VydC5jb20vVmVyb2tleVNlY3VyZUVtYWlsRzIu Y3JsMDegNaAzhjFodHRwOi8vY3JsNC5kaWdpY2VydC5jb20vVmVyb2tleVNlY3VyZUVtYWlsRzIu Y3JsMHYGCCsGAQUFBwEBBGowaDAkBggrBgEFBQcwAYYYaHR0cDovL29jc3AuZGlnaWNlcnQuY29t MEAGCCsGAQUFBzAChjRodHRwOi8vY2FjZXJ0cy5kaWdpY2VydC5jb20vVmVyb2tleVNlY3VyZUVt YWlsRzIuY3J0MA0GCSqGSIb3DQEBCwUAA4IBAQBBdzgU+I8tGdMO+Y4AETOQi6aiN9kB7lKWe5Ch 4VR+L4uxYIqIHxR7G2+IEC9ugqEu43p4b80LpY7M4KmmfiaRDFGQ50s1OHAwdbR3X2OtkUQq0Qb9 6ln+HD8N1JxO5nabuKbymki0BPoZcPdo+FeRecmkL/NOzzm41JBHrhwRwEWOOhAO5KxN4nkMBZ/w QzKEy4PylxurHmRG/K0k+xYFDO/UOx2/YsM8s138lQqEdKCvudtSvj5oA/Y8dNcZwQGHyVN5h5r2 nh3mT3r2l7Q4dgxXlovERGpNqCZJ624jCiWQC4ELMD2+6WDxjj03PbOulQZ8oY4PQUyp6djF0keA MYIDuzCCA7cCAQEwVTBBMQswCQYDVQQGEwJBVTEQMA4GA1UEChMHVmVyb2tleTEgMB4GA1UEAxMX VmVyb2tleSBTZWN1cmUgRW1haWwgRzICEAfkkQ9qA1FdgOJE92VzW+AwDQYJYIZIAWUDBAIBBQCg ggE3MBgGCSqGSIb3DQEJAzELBgkqhkiG9w0BBwEwHAYJKoZIhvcNAQkFMQ8XDTI2MDIyMzE5NTcx MFowLwYJKoZIhvcNAQkEMSIEIBCJeIuHqMNbOH5frCxBcy5jw5wZMHNkFaRCRzPpCrgvMGQGCSsG AQQBgjcQBDFXMFUwQTELMAkGA1UEBhMCQVUxEDAOBgNVBAoTB1Zlcm9rZXkxIDAeBgNVBAMTF1Zl cm9rZXkgU2VjdXJlIEVtYWlsIEcyAhAH5JEPagNRXYDiRPdlc1vgMGYGCyqGSIb3DQEJEAILMVeg VTBBMQswCQYDVQQGEwJBVTEQMA4GA1UEChMHVmVyb2tleTEgMB4GA1UEAxMXVmVyb2tleSBTZWN1 cmUgRW1haWwgRzICEAfkkQ9qA1FdgOJE92VzW+AwDQYJKoZIhvcNAQEBBQAEggIABi8unOXAaLjy UqD18uwy+6xdWr8bTD5JyhrD3pyNha9XaLUD8o32oIiye9i5c9cbZ2lvuqIaZOFYdrdKBOFcbjqs tBizvj5yvYpU3oiilI3y5BAggrRxdmRRbJXYyR1MyDtGRNe+6CTJp798Cwpcv2Yv2kO06GHf9bjR f+GH4cro2CHd8IgBOB5huABgdiWxzyePhaowAw8hWjxG1q/if+k7Bq67wniVL5n+rD4NPd4dEGeI VjvuP6S1Lcdt27bNHZuOV2iWxxZH3AJl3+cHjzv46rs+A8tMrQpfIsTVzhGbGt/YU+spTAkzuXJb lB09KC66wZVCD4RHngNNVro1iA1Q50ai0nCbCErDkQ0inc6XMnKHzg0GJLM6Zawi1vRpuiZxS8oO hmPSsSuQtkC6cW57wxF1pp2L2L9DF3dVMTFgD2u+F7Mn/5PqO5M3kpJKVdj81GftVCUnfDyGDLLq ihzjyKtvc1GsdRm33GpovhctvhYU4EHHnG915LFEbERlcSyl6e/osWBYclzr9SiQ2neAbrQShCu0 IfyEh7A25fH6azrZX0feSDKD0lHh2iSjV0DZQ1CfobVVMdxjk6Fz/eWxi3cUzqaSoAiO2eW3m6G1 j9FMX5cYGiOieT77mT0KhtZvkosi8pQ5eQw3Jiw/5uAt967jVHo2e/16AEpDAx4AAAAAAAA= --=-a69pBfocK4srBbC7syIY--