/* 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

*/

 

#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[2];

// surely writev doesn't write to the segment

vec[0].iov_base = (char *) msg;

vec[0].iov_len = strlen(msg);

vec[1].iov_base = "\n";

vec[1].iov_len = 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;

}