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If a lower device is found, we don't need to subtract LL_MAX_HEADER to calculate our MTU: just use its MTU, the link layer headers are already taken into account by it. If the lower device is not found, start from ETH_DATA_LEN instead, and only in this case subtract a worst-case LL_MAX_HEADER. We then need to subtract our additional IPv6 header from the calculation. While at it, note that vti6 doesn't have a hardware header, so it doesn't need to set dev->hard_header_len. And as vti6_link_config() now always sets the MTU, there's no need to set a default value in vti6_dev_setup(). This makes the behaviour consistent with IPv4 vti, after commita32452366b("vti4: Don't count header length twice."), which was accidentally reverted by merge commitf895f0cfbb("Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/klassert/ipsec"). While commit53c81e95df("ip6_vti: adjust vti mtu according to mtu of lower device") improved on the original situation, this was still not ideal. As reported in that commit message itself, if we start from an underlying veth MTU of 9000, we end up with an MTU of 8832, that is, 9000 - LL_MAX_HEADER - sizeof(ipv6hdr). This should simply be 8880, or 9000 - sizeof(ipv6hdr) instead: we found the lower device (veth) and we know we don't have any additional link layer header, so there's no need to subtract an hypothetical worst-case number. Fixes:53c81e95df("ip6_vti: adjust vti mtu according to mtu of lower device") Signed-off-by: Stefano Brivio <sbrivio@redhat.com> Acked-by: Sabrina Dubroca <sd@queasysnail.net> Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.