awkwrapper.c

/* awkwrapper: minimal accept-and-fork loop for smtpd.awk when it's not
 * run directly through systemd (with StandardInput=socket & Accept=true) or
 * inetd, but using socket activation
 */

// only really needed for strdupa()
#define _GNU_SOURCE
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/uio.h>
#include <unistd.h>
#include <fcntl.h>

#define LISTEN_FD 3
static const char SCRIPT_NAME[] = "/usr/local/bin/smtpd.awk";

static int punt(const char *) __attribute__ ((noreturn));

static int punt(const char *msg) {
    struct iovec vec[] = {
        // surely writev doesn't write to the segment
        { (char *) msg, strlen(msg) },
        { "\n", 1 }
    };
    writev(2, vec, 2);
    exit(1);
}

static void setup_socket_activation() {
    char buf[20];
    char *expected;
    pid_t how_long_is_this = getpid();
    if (!getenv("LISTEN_FDS")) punt("No socket activation");
    if ((expected = getenv("LISTEN_PID")) != NULL) {
        buf[19] = 0;
        int i;
        for (i = 18; i >= 0; --i) {
            buf[i] = '0' + how_long_is_this % 10;
            how_long_is_this /= 10;
            if (!how_long_is_this) break;
        }
        if (strcmp(expected, &buf[i]) != 0) punt("Wrong process");
    } else {
        punt("No socket activation");
    }
    unsetenv("LISTEN_FDS");
    unsetenv("LISTEN_FDNAMES");
    unsetenv("LISTEN_PID");
    unsetenv("LISTEN_PIDFDID");
}

static int fork_connection(int fd) {
    switch (fork()) {
    case -1:
        return -1;
    case 0:
        if ((dup2(fd, 0) == -1) || (dup2(fd, 1) == -1)) punt("child: dup2 failed");
        close(fd);
        char *argv[] = { strdupa(SCRIPT_NAME), (char *) NULL };
        execv(SCRIPT_NAME, argv);
        punt("child: execv failed");
    default:
        close(fd);
        return 0;
    }
}

int main() {
    struct sigaction ignore_child = { .sa_handler = SIG_IGN, .sa_flags = SA_NOCLDSTOP };
    setup_socket_activation();
    if (sigaction(SIGCHLD, &ignore_child, NULL) == -1) punt("sigaction failed");
    if (fcntl(LISTEN_FD, F_SETFD, FD_CLOEXEC) == -1) punt("close-on-exec failed, probably not a socket");
    while (1) {
        int fd = accept(LISTEN_FD, NULL, NULL);
        if (fd == -1) {
            if (errno != EINTR) punt("accept failed");
            else continue;
        }
        if (fork_connection(fd) == -1) {
            punt("fork failed, better exit before something terrible happens");
        }
    }
    // just in case we somehow break out of the loop
    return 0;
}