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Fri, 04 Sep 2026 14:00:28 -0700 (PDT) From: Janis Edvarts Lacis To: netdev@vger.kernel.org Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, linux-kernel@vger.kernel.org, Janis Edvarts Lacis Subject: [PATCH net] net: vlan: fix vlan name truncation Date: Sat, 5 Sep 2026 00:00:24 +0300 Message-ID: <20260904210024.263220-1-janislacis06@gmail.com> X-Mailer: git-send-email 2.43.0 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=UTF-8 Content-Transfer-Encoding: 8bit register_vlan_device() from net/8021q/vlan.c sets name based on a field name_type from struct vlan_net. If name_type is VLAN_NAME_TYPE_RAW_PLUS_VID or VLAN_NAME_TYPE_RAW_PLUS_VID_NO_PAD, register_vlan_device() sets vlan device's name to the actual net_device's name and adds vid at the end of this name separated by a dot. Network interface's name is limited to at most 16 characters, including NUL. Therefore, if the real_dev->name is long enough, then adding the vid at the end of this name can lead to the result exceeding 16 bytes which leads to a truncated name for the vlan device created in register_vlan_device(). This patch fixes the issue by returning -ENAMETOOLONG from register_vlan_device() if the newly created name does not fit in 16 bytes. The bug was discovered when compiling with flag W=1, the following warning comes up: linux/net/8021q/vlan.c: In function ‘vlan_ioctl_handler’: linux/net/8021q/vlan.c:250:46: error: ‘%i’ directive output may be truncated writing between 1 and 5 bytes into a region of size between 0 and 15 [-Werror=format-truncation=] 250 | snprintf(name, IFNAMSIZ, "%s.%i", real_dev->name, vlan_id); | ^~ In function ‘register_vlan_device’, inlined from ‘vlan_ioctl_handler’ at linux/net/8021q/vlan.c:588:9: linux/net/8021q/vlan.c:250:42: note: directive argument in the range [0, 65535] 250 | snprintf(name, IFNAMSIZ, "%s.%i", real_dev->name, vlan_id); | ^~~~~~~ Build & run the reproducer: gcc poc.c -o poc chmod +x poc.sh ./poc.sh ======BEGIN poc.sh====== #!/usr/bin/env bash sudo ip link add vlan1234567890 type dummy sudo ./poc # name will not match the expected printed from poc.c on the unpatched # kernel, there will be no vlan interface at all on the patched one ip a ======END poc.sh======== ======BEGIN poc.c====== #include #include #include #include #include #include #include #define IF_NAME "vlan1234567890" int main() { struct vlan_ioctl_args args = { .cmd = ADD_VLAN_CMD, .u.VID = 1234, }; strcpy(args.device1, IF_NAME); int fd = socket(AF_INET, SOCK_STREAM, 0); if (fd < 0) { perror("socket"); return 1; } /* fails with -ENAMETOOLONG */ if (ioctl(fd, SIOCSIFVLAN, &args) < 0) { perror("SIOCSIFVLAN"); close(fd); return 1; } printf("Expected name: %s.%i\n", IF_NAME, args.u.VID); close(fd); return 0; } ======END poc.c======== Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Janis Edvarts Lacis --- net/8021q/vlan.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c index 2d2efb877975..b51092d217de 100644 --- a/net/8021q/vlan.c +++ b/net/8021q/vlan.c @@ -220,7 +220,7 @@ static int register_vlan_device(struct net_device *real_dev, u16 vlan_id) struct net *net = dev_net(real_dev); struct vlan_net *vn = net_generic(net, vlan_net_id); char name[IFNAMSIZ]; - int err; + int err, len; if (vlan_id >= VLAN_VID_MASK) return -ERANGE; @@ -234,7 +234,9 @@ static int register_vlan_device(struct net_device *real_dev, u16 vlan_id) switch (vn->name_type) { case VLAN_NAME_TYPE_RAW_PLUS_VID: /* name will look like: eth1.0005 */ - snprintf(name, IFNAMSIZ, "%s.%.4i", real_dev->name, vlan_id); + len = snprintf(name, IFNAMSIZ, "%s.%.4i", real_dev->name, vlan_id); + if (len >= sizeof(name)) + err = -ENAMETOOLONG; break; case VLAN_NAME_TYPE_PLUS_VID_NO_PAD: /* Put our vlan.VID in the name. @@ -246,7 +248,9 @@ static int register_vlan_device(struct net_device *real_dev, u16 vlan_id) /* Put our vlan.VID in the name. * Name will look like: eth0.5 */ - snprintf(name, IFNAMSIZ, "%s.%i", real_dev->name, vlan_id); + len = snprintf(name, IFNAMSIZ, "%s.%i", real_dev->name, vlan_id); + if (len >= sizeof(name)) + err = -ENAMETOOLONG; break; case VLAN_NAME_TYPE_PLUS_VID: /* Put our vlan.VID in the name. @@ -256,6 +260,9 @@ static int register_vlan_device(struct net_device *real_dev, u16 vlan_id) snprintf(name, IFNAMSIZ, "vlan%.4i", vlan_id); } + if (err) + return err; + new_dev = alloc_netdev(sizeof(struct vlan_dev_priv), name, NET_NAME_UNKNOWN, vlan_setup); -- 2.43.0