Various userspace programs (e.g. iproute2) have sent RTM_GETADDR requests with struct ifinfomsg. This is wrong and should have been struct ifaddrmsg all along as mandated by the manpages. However, dump requests so far didn't parse the netlink message that was sent and succeeded even when a wrong struct was passed along.
Currently, the message is parsed under the assumption that the correct struct ifaddrmsg is sent down. If the parsing fails the kernel will still fulfill the request to preserve backwards compatability but a rate-limited message that there were leftover bytes after parsing the message is recorded in dmesg. It has been argued that this is unacceptable [1]. But various new features that got and will get added to RTM_GETADDR make it necessary to definitely know what header was passed along. This is currently not possible without resorting to (likely unreliable) hacks such as introducing a nested attribute that ensures that RTM_GETADDR which pass along properties such as IFA_TARGET_NETNSID always exceed RTM_GETADDR requests that send down the wrong struct ifinfomsg [2]. Basically, multiple approaches to solve this have been shut down. Furthermore, the API expressed via RTM_GETADDR is apparently frozen [3] so the wiggle room at this point seems very much zero. The correct solution at this point seems to me to introduce a new RTM_GETADDR2 request. This way we can parse the message and fail hard if the struct is not struct ifaddrmsg and can safely extend it in the future. Userspace tools that rely on the buggy RTM_GETADDR API will still keep working without even having to see any log messages and new userspace tools that want to make user of new features can make use of the new RTM_GETADDR2 requests. [1]: https://lists.openwall.net/netdev/2018/09/25/59 [2]: https://lists.openwall.net/netdev/2018/09/25/75 [3]: https://lists.openwall.net/netdev/2018/09/26/166 Signed-off-by: Christian Brauner <christ...@brauner.io> Cc: David Ahern <dsah...@gmail.com> Cc: Jiri Benc <jb...@redhat.com> Cc: Stephen Hemminger <step...@networkplumber.org> --- include/uapi/linux/rtnetlink.h | 3 +++ 1 file changed, 3 insertions(+) diff --git a/include/uapi/linux/rtnetlink.h b/include/uapi/linux/rtnetlink.h index 46399367627f..e167f90c3d7a 100644 --- a/include/uapi/linux/rtnetlink.h +++ b/include/uapi/linux/rtnetlink.h @@ -157,6 +157,9 @@ enum { RTM_GETCHAIN, #define RTM_GETCHAIN RTM_GETCHAIN + RTM_GETADDR2 = 106, +#define RTM_GETADDR2 RTM_GETADDR2 + __RTM_MAX, #define RTM_MAX (((__RTM_MAX + 3) & ~3) - 1) }; -- 2.17.1
