https://patchwork.ozlabs.org/patch/579654
lists this as 'superseded', among with the older versions of the patch which changed the error handling. But at least, I couldn't find anything superseding it. This was supposed to address the different-but-related problem demonstrated by the following (slightly modified) test program: --------- #define _GNU_SOURCE #include <stdlib.h> #include <stdio.h> #include <sys/socket.h> #include <sys/stat.h> #include <assert.h> #include <errno.h> #include <string.h> #include <unistd.h> int main(void) { enum { server, client, size }; int socket_fd[size]; int const opt = 1; assert(socketpair(AF_LOCAL, SOCK_STREAM, 0, socket_fd) == 0); assert(setsockopt(socket_fd[server], SOL_SOCKET, SO_PASSCRED, &opt, sizeof(opt)) != -1); char const msg[] = "A random message"; if (fork() == 0) { sleep(1); send(socket_fd[client], msg, sizeof msg, 0); _exit(0); } union { struct cmsghdr cmh; char control[CMSG_SPACE(sizeof(struct ucred))]; } control_un; control_un.cmh.cmsg_len = CMSG_LEN(sizeof(struct ucred)); control_un.cmh.cmsg_level = SOL_SOCKET; control_un.cmh.cmsg_type = SCM_CREDENTIALS; struct msghdr msgh; msgh.msg_name = NULL; msgh.msg_namelen = 0; msgh.msg_iov = NULL; msgh.msg_iovlen = 0; msgh.msg_control = control_un.control; msgh.msg_controllen = sizeof(control_un.control); recvmsg(socket_fd[server], &msgh, MSG_PEEK); printf("Success?\n"); struct ucred *ucred; ucred = (void *)CMSG_DATA(&control_un.cmh); printf("... pid %ld, uid %d, gid %d\n", (long)ucred->pid, ucred->uid, ucred->gid); return 0; } -------- Because the receiver has to wait for the message, it will hit the continue in unix_stream_read_generic. This causes the size-check of the do-while loop to be executed which terminates the loop as the size is zero without copying the credential information. Just wondering if this might have been lost in the noise ...