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 F14D02C18A; Wed, 3 Sep 2025 19:36:20 +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=1756928181; cv=none; b=ndt3RZhfYgjZsb67SmWap6oCzHILQakFNQuoPDbY6rjNx+gKbH/KVz7mFxahfs9T8H8KjjTFArtkHC5SoyQABR0sADydvPZeCIBPsPSxODoTPDhdTchN1uwfbCVDLPxy2piTEr2nOM7GBf3wtZ0tNqgMaQdzriISvWuQAzu89w8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756928181; c=relaxed/simple; bh=J5VDjAzU4bP2iNAyELFb3jm1tCgK9MjUGPIE6C22Gdg=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition; b=u3rU247m8p6EeBeU9o817R75aAV5ckOgzc6GzfM4io+UXXBAKRkzb5EHh46R5fKTbYf2KT/fppZSZV4k324/wkSINpkUkRWvsxugJw7FGtbv+lT2ujpayeDXXx6/giaDMWwzRo0v9iXj4BKA4qcbjiFJvSGbvzbTai/C3IPOEaY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ocaVzao4; 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="ocaVzao4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E3982C4CEE7; Wed, 3 Sep 2025 19:36:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1756928180; bh=J5VDjAzU4bP2iNAyELFb3jm1tCgK9MjUGPIE6C22Gdg=; h=Date:From:To:Cc:Subject:From; b=ocaVzao4pHj7eKEMcbd5Gjt92N6UK53kmHR2XF447/zhQoXmw8ED4iAm8ilUtAevr vKKdTzgnJB9FZ1a+PVsxADdMDORDhph18ZzFI/AdYmCUQTYzzC/Cz/GKlZS9k1Zp9e NUT1+eg71S5VoVdEIEVAU4AaUwVtZPQXN2u+rLVMtT5EMAtSknA7fwUlQx8dAotCLT U9zjAzodQ6z5ATB5gzCWpsQe4EnD+a1Dzt2oh3l4QiKDfzIiGjs5jXyKiKt1azlZ9F L5BJUGaHOfxeUz/4t2P+LZ3uQGRmemu7XC5xnuUYUudbHtyjsMRq9/ZdI0Zq9DwdS5 /SI2PRaNEUmdQ== Date: Wed, 3 Sep 2025 21:36:13 +0200 From: "Gustavo A. R. Silva" To: "Michael S. Tsirkin" , Jason Wang , Xuan Zhuo , Eugenio =?iso-8859-1?Q?P=E9rez?= , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "Gustavo A. R. Silva" , linux-hardening@vger.kernel.org Subject: [PATCH][next] virtio_net: Fix alignment and avoid -Wflex-array-member-not-at-end warning Message-ID: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline -Wflex-array-member-not-at-end was introduced in GCC-14, and we are getting ready to enable it, globally. Use the new TRAILING_OVERLAP() helper to fix the following warning: drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] This helper creates a union between a flexible-array member (FAM) and a set of members that would otherwise follow it (in this case `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This overlays the trailing members (rss_hash_key_data) onto the FAM (hash_key_data) while keeping the FAM and the start of MEMBERS aligned. The static_assert() ensures this alignment remains, and it's intentionally placed inmediately after `struct virtnet_info` (no blank line in between). Notice that due to tail padding in flexible `struct virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data` (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at offset 84 in struct virtnet_info) are misaligned by one byte. See below: struct virtio_net_rss_config_trailer { __le16 max_tx_vq; /* 0 2 */ __u8 hash_key_length; /* 2 1 */ __u8 hash_key_data[]; /* 3 0 */ /* size: 4, cachelines: 1, members: 3 */ /* padding: 1 */ /* last cacheline: 4 bytes */ }; struct virtnet_info { ... struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */ /* XXX last struct has 1 byte of padding */ u8 rss_hash_key_data[40]; /* 84 40 */ ... /* size: 832, cachelines: 13, members: 48 */ /* sum members: 801, holes: 8, sum holes: 31 */ /* paddings: 2, sum paddings: 5 */ }; After changes, those members are correctly aligned at offset 795: struct virtnet_info { ... union { struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */ struct { unsigned char __offset_to_hash_key_data[3]; /* 792 3 */ u8 rss_hash_key_data[40]; /* 795 40 */ }; /* 792 43 */ }; /* 792 44 */ ... /* size: 840, cachelines: 14, members: 47 */ /* sum members: 801, holes: 8, sum holes: 35 */ /* padding: 4 */ /* paddings: 1, sum paddings: 4 */ /* last cacheline: 8 bytes */ }; As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also moved to the end, since it seems those three members should stick around together. :) Signed-off-by: Gustavo A. R. Silva --- This should probably include the following tag: Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs") but I'd like to hear some feedback, first. Thanks! drivers/net/virtio_net.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 975bdc5dab84..f4964a18a214 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -425,9 +425,6 @@ struct virtnet_info { u16 rss_indir_table_size; u32 rss_hash_types_supported; u32 rss_hash_types_saved; - struct virtio_net_rss_config_hdr *rss_hdr; - struct virtio_net_rss_config_trailer rss_trailer; - u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE]; /* Has control virtqueue */ bool has_cvq; @@ -493,7 +490,16 @@ struct virtnet_info { struct failover *failover; u64 device_stats_cap; + + struct virtio_net_rss_config_hdr *rss_hdr; + + /* Must be last --ends in a flexible-array member. */ + TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data, + u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE]; + ); }; +static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) == + offsetof(struct virtnet_info, rss_hash_key_data)); struct padded_vnet_hdr { struct virtio_net_hdr_v1_hash hdr; -- 2.43.0