Hi! I'd like to ask about SCM credentials for netlink, and the meaning of pid == 0 in particular. SCM credentials are documented for AF_UNIX in unix(7), see "Ancillary messages". It says "they are AF_UNIX specific". However, the Linux kernel supports them for AF_NETLINK too. Would it be reasonable to mention SCM credentials in netlink(7) as it is done in unix(7)? The next part is about the meaning of nl_pid (netlink port id) == 0 versus credentials pid == 0 in netlink packets. There was a mail [1] from Eric Biederman saying "If in the senders address nl_pid == 0 then the message is from the kernel. [...] Looking anywhere else at anything else is bogus." Some user space tools ([2], [3]) assert SCM credentials pid == 0 instead of checking nl_pid. So I'm curious if using SCM credentials for this purpose is bogus, and why? AFAICT credentials pid == 0 means a netlink packet is from kernel: -there was a kernel patch "Force passing credentials if none given", to guarantee credentials are always included in netlink packets [4] -task_tgid(current) is assigned to pid, which is 0 for all kernel threads -it works within user and network namespaces Tobias [1] https://lists.linuxfoundation.org/pipermail/containers/2015-May/036032.html [2] https://github.com/lathiat/avahi/blob/6242e5f0fe001b7de2ccaa9431db279b2ee76b83/avahi-core/netlink.c#L83-L86 [3] https://github.com/NetworkManager/NetworkManager/blob/4650a4e1a1ff7ee03e2fb0b3dc6f31b911c2583d/src/platform/nm-linux-platform.c#L6666-L6673 [4] git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e0e3cea46d31d23dc40df0a49a7a2c04fe8edfea -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
