+ if (created) fd = Uopen(name, flags, LOG_MODE);
+ }
+
+return fd;
+}
+
+
+
+/* Inspired by OpenSSH's mm_send_fd(). Thanks!
+Send fd over socketpair.
+Return: true iff good.
+*/
+
+static BOOL
+log_send_fd(const int sock, const int fd)
+{
+struct msghdr msg;
+union {
+ struct cmsghdr hdr;
+ char buf[CMSG_SPACE(sizeof(int))];
+} cmsgbuf;
+struct cmsghdr *cmsg;
+char ch = 'A';
+struct iovec vec = {.iov_base = &ch, .iov_len = 1};
+ssize_t n;
+
+memset(&msg, 0, sizeof(msg));
+memset(&cmsgbuf, 0, sizeof(cmsgbuf));
+msg.msg_control = &cmsgbuf.buf;
+msg.msg_controllen = sizeof(cmsgbuf.buf);
+
+cmsg = CMSG_FIRSTHDR(&msg);
+cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+cmsg->cmsg_level = SOL_SOCKET;
+cmsg->cmsg_type = SCM_RIGHTS;
+*(int *)CMSG_DATA(cmsg) = fd;
+
+msg.msg_iov = &vec;
+msg.msg_iovlen = 1;
+
+while ((n = sendmsg(sock, &msg, 0)) == -1 && errno == EINTR);
+return n == 1;
+}
+
+/* Inspired by OpenSSH's mm_receive_fd(). Thanks!
+Return fd passed over socketpair, or -1 on error.
+*/
+
+static int
+log_recv_fd(const int sock)
+{
+struct msghdr msg;
+union {
+ struct cmsghdr hdr;
+ char buf[CMSG_SPACE(sizeof(int))];
+} cmsgbuf;
+struct cmsghdr *cmsg;
+char ch = '\0';
+struct iovec vec = {.iov_base = &ch, .iov_len = 1};
+ssize_t n;
+int fd;
+
+memset(&msg, 0, sizeof(msg));
+msg.msg_iov = &vec;
+msg.msg_iovlen = 1;
+
+memset(&cmsgbuf, 0, sizeof(cmsgbuf));
+msg.msg_control = &cmsgbuf.buf;
+msg.msg_controllen = sizeof(cmsgbuf.buf);
+
+while ((n = recvmsg(sock, &msg, 0)) == -1 && errno == EINTR) ;
+if (n != 1 || ch != 'A') return -1;
+
+if (!(cmsg = CMSG_FIRSTHDR(&msg))) return -1;
+if (cmsg->cmsg_type != SCM_RIGHTS) return -1;
+if ((fd = *(const int *)CMSG_DATA(cmsg)) < 0) return -1;
+return fd;
+}
+
+
+
+/*************************************************
+* Create a log file as the exim user *
+*************************************************/
+
+/* This function is called when we are root to spawn an exim:exim subprocess
+in which we can create a log file. It must be signal-safe since it is called
+by the usr1_handler().
+
+Arguments:
+ name the file name
+
+Returns: a file descriptor, or < 0 on failure (errno set)
+*/
+
+int
+log_open_as_exim(uschar * const name)
+{
+int fd = -1;
+const uid_t euid = geteuid();
+
+if (euid == exim_uid)
+ fd = log_open_already_exim(name);
+else if (euid == root_uid)
+ {
+ int sock[2];
+ if (socketpair(AF_UNIX, SOCK_STREAM, 0, sock) == 0)
+ {
+ const pid_t pid = fork();
+ if (pid == 0)
+ {
+ (void)close(sock[0]);
+ if ( setgroups(1, &exim_gid) != 0
+ || setgid(exim_gid) != 0
+ || setuid(exim_uid) != 0
+
+ || getuid() != exim_uid || geteuid() != exim_uid
+ || getgid() != exim_gid || getegid() != exim_gid
+
+ || (fd = log_open_already_exim(name)) < 0
+ || !log_send_fd(sock[1], fd)
+ ) _exit(EXIT_FAILURE);
+ (void)close(sock[1]);
+ _exit(EXIT_SUCCESS);
+ }
+
+ (void)close(sock[1]);
+ if (pid > 0)
+ {
+ fd = log_recv_fd(sock[0]);
+ while (waitpid(pid, NULL, 0) == -1 && errno == EINTR);
+ }
+ (void)close(sock[0]);
+ }