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Tue, 13 Jan 2026 07:06:07 -0800 (PST) Received: from [192.168.88.32] ([212.105.155.93]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47ee1189f5fsm2255445e9.2.2026.01.13.07.06.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 13 Jan 2026 07:06:06 -0800 (PST) Message-ID: <916a6c1a-681e-4e5e-8d49-75d0de5c46a1@redhat.com> Date: Tue, 13 Jan 2026 16:06:05 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2][next] virtio_net: Fix misalignment bug in struct virtnet_info To: "Michael S. Tsirkin" , Jakub Kicinski Cc: "Gustavo A. R. Silva" , Jason Wang , Xuan Zhuo , =?UTF-8?Q?Eugenio_P=C3=A9rez?= , Andrew Lunn , "David S. Miller" , Eric Dumazet , Akihiko Odaki , Simon Horman , netdev@vger.kernel.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, Kees Cook References: <20260113093839-mutt-send-email-mst@kernel.org> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20260113093839-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 1/13/26 3:39 PM, Michael S. Tsirkin wrote: > On Tue, Jan 13, 2026 at 03:30:00PM +0100, Paolo Abeni wrote: >> On 1/10/26 9:07 AM, Gustavo A. R. Silva wrote: >>> Use the new TRAILING_OVERLAP() helper to fix a misalignment bug >>> along with 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. >>> >>> 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 result, the RSS key passed to the device is shifted by 1 >>> byte: the last byte is cut off, and instead a (possibly >>> uninitialized) byte is added at the beginning. >>> >>> 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. :) >>> >>> Cc: stable@vger.kernel.org >>> Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs") >>> Signed-off-by: Gustavo A. R. Silva >>> --- >>> Changes in v2: >>> - Update subject and changelog text (include feedback from Simon and >>> Michael --thanks folks) >>> - Add Fixes tag and CC -stable. >> >> @Michael, @Jason: This is still apparently targeting 'net-next', but I >> think it should land in the 'net' tree, right? >> >> /P > > Probably but I'm yet to properly review it. The thing that puzzles me at > a first glance is how are things working right now then? Apparently they aren't ?!? rss self-tests for virtio_net are failing: https://netdev-ctrl.bots.linux.dev/logs/vmksft/drv-hw-dbg/results/471521/15-rss-api-py/stdout but the result is into the CI reported as success (no idea why?!?) /P