conf: allow --fd 0

inetd-style socket passing traditionally starts a service with a
connected socket on file descriptors 0 and 1.  passt disallowing
obtaining its socket from either of these descriptors made it
difficult to use with super-servers providing this interface — in my
case I wanted to use passt with s6-ipcserver[1].  Since (as far as I
can tell) passt does not use standard input for anything else (unlike
standard output), it should be safe to relax the restrictions on --fd
to allow setting it to 0, enabling this use case.

Link: https://skarnet.org/software/s6/s6-ipcserver.html [1]
Signed-off-by: Alyssa Ross <hi@alyssa.is>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -1717,7 +1717,8 @@ void conf(struct ctx *c, int argc, char **argv)
 			fd_tap_opt = strtol(optarg, NULL, 0);
 
 			if (errno ||
-			    fd_tap_opt <= STDERR_FILENO || fd_tap_opt > INT_MAX)
+			    (fd_tap_opt != STDIN_FILENO && fd_tap_opt <= STDERR_FILENO) ||
+			    fd_tap_opt > INT_MAX)
 				die("Invalid --fd: %s", optarg);
 
 			c->fd_tap = fd_tap_opt;

util.c

@@ -875,7 +875,9 @@ void close_open_files(int argc, char **argv)
 			errno = 0;
 			fd = strtol(optarg, NULL, 0);
 
-			if (errno || fd <= STDERR_FILENO || fd > INT_MAX)
+			if (errno ||
+			    (fd != STDIN_FILENO && fd <= STDERR_FILENO) ||
+			    fd > INT_MAX)
 				die("Invalid --fd: %s", optarg);
 		}
 	} while (name != -1);