die with sensible error message if socket path too long

README.md

@@ -17,6 +17,15 @@ Liminix, but it may be useful elsewhere too.
 * accept unix datagram socket commands
 * fork a child for each peer socket and run the requested lua script
 
+## how to use it
+
+    $ LUAD_SOCKET_PATH=/var/run/luad/socket luad prelude.lua
+
+    $ LUAD_SOCKET_PATH=/var/run/luad/socket luac script.lua arg1 arg2 ...
+
+The pathname of a Lua script may optionally be passed to luad. It will
+be loaded into the interpreter before it is forked, so whatever it
+does will be available in the children.
 
 ## small print
 
@@ -25,8 +34,3 @@ Liminix, but it may be useful elsewhere too.
 * tested only on Linux. Needs SCM_RIGHTS to send unix file descriptors
   across processes, which seems to be have [implementation quirks on
   different unix flavours](https://gist.github.com/kentonv/bc7592af98c68ba2738f4436920868dc)
-
-
-## TO DO
-
-[.] implement luac

luac.c

@@ -12,6 +12,11 @@
 
 #include <arpa/inet.h>
 
+#include <error.h>
+#include <errno.h>
+
+#define die(...) error(EXIT_FAILURE, errno, __VA_ARGS__)
+
 // copied from OpenBSD
 // https://github.com/openbsd/src/blob/master/lib/libc/string/strlcpy.c
 
@@ -47,23 +52,20 @@ size_t strlcpy(char *dst, const char *src, size_t dsize)
 int open_socket(struct sockaddr_un *addr, struct sockaddr_un * peer_addr) {
     int fd = socket(AF_LOCAL, SOCK_DGRAM, PF_LOCAL);
     if(fd < 0) {
-        perror("socket");
-        exit(EXIT_FAILURE);
+        die("can't create socket");
     }
 
     // local address is in the abstract namespace and ok to include
     // trailing nul bytes
     if((bind(fd, (struct sockaddr *) addr, sizeof (struct sockaddr_un))) < 0) {
-        perror("bind");
-        exit(EXIT_FAILURE);
+        die("can't bind socket local address");
     }
 
     // peer address is in the actual filesystem, so we need to pass
     // the real length
     int namelen = sizeof peer_addr->sun_family + strlen(peer_addr->sun_path);
     if((connect(fd, (struct sockaddr *) peer_addr, namelen)) < 0) {
-      perror("connect");
-      exit(EXIT_FAILURE);
+        die("can't connect socket to \"%s\"", peer_addr->sun_path);
     }
 
     return fd;
@@ -76,8 +78,7 @@ int encode_args(char * buf, int len, int argc, char ** argv) {
     for (int i = 1; i < argc; i++) {
         p += strlcpy(p, argv[i], buf_end - p);
         if(p >= buf_end) {
-            fprintf(stderr, "comand line too long\n");
-            exit(1);
+            error(1, 0, "comand line too long");
         }
         p++;
     }
@@ -102,6 +103,11 @@ int main(int argc, char **argv)
         exit(1);
     }
 
+    char * socket_path = getenv("LUAD_SOCKET_PATH");
+    if(! socket_path) {
+        error(1, 0, "Missing required environment variable LUAD_SOCKET_PATH");
+    }
+
     char * iobuf = (char *) calloc(1024, sizeof(char));
     if(!iobuf) return 1;
     int iobuf_len = encode_args(iobuf, 1024, argc, argv);
@@ -117,7 +123,8 @@ int main(int argc, char **argv)
     memcpy(my_addr.sun_path + 1, (void *) &mypid, sizeof mypid);
     memcpy(my_addr.sun_path + 1 + sizeof mypid, iobuf, maxnamelen - sizeof mypid - 1);
 
-    struct sockaddr_un peer_addr = { AF_UNIX, "/tmp/mysock" };
+    struct sockaddr_un peer_addr = { AF_UNIX };
+    strlcpy(peer_addr.sun_path, socket_path, sizeof peer_addr.sun_path);
     int socket = open_socket(&my_addr, &peer_addr);
 
     struct cmsghdr *cmsg;
@@ -147,10 +154,8 @@ int main(int argc, char **argv)
     cmsg->cmsg_len = CMSG_LEN(sizeof(myfds));
     memcpy(CMSG_DATA(cmsg), myfds, sizeof(myfds));
 
-    int bytes = sendmsg(socket, &msg, 0);
-    if(bytes <= 0) {
-        perror("sendmsg");
-        exit(1);
+    if(sendmsg(socket, &msg, 0) <= 0) {
+        die("sendmsg failed");
     }
 
     uint32_t exitstatus = 0;

luad.c

@@ -8,6 +8,8 @@
 #include <string.h>
 #include <stdlib.h>
 #include <stdint.h>
+#include <errno.h>
+#include <error.h>
 #include <sys/un.h>
 
 #include <signal.h>
@@ -25,22 +27,24 @@ static void handle_sigchld(int num) {
     }
 }
 
+#define die(...) error(EXIT_FAILURE, errno, __VA_ARGS__)
 
 int open_socket(char * pathname) {
-
     struct sockaddr_un addr;
     addr.sun_family = AF_UNIX;
-    strncpy(addr.sun_path, pathname, sizeof(addr.sun_path) - 1);
+    if(strlen(pathname) > sizeof(addr.sun_path)) {
+        die("pathname \"%s\" is too long to  bind as unix socket (max %lu)",
+            pathname, sizeof(addr.sun_path));
+    }
+    strcpy(addr.sun_path, pathname);
 
     int fd = socket(AF_LOCAL, SOCK_DGRAM, PF_LOCAL);
     if(fd < 0) {
-        perror("socket");
-        exit(EXIT_FAILURE);
+        die("can't create socket \"%s\"", pathname);
     }
 
     if((bind(fd, (struct sockaddr *) &addr, sizeof addr)) < 0) {
-        perror("bind");
-        exit(EXIT_FAILURE);
+        die("can't bind socket \"%s\"", pathname);
     }
     return fd;
 }
@@ -103,8 +107,7 @@ pid_t fork_lua(lua_State *L,
         free(command_line);
         return pid;
     } else {
-        perror("fork");
-        return -1;
+        die("fork failed");
     }
 }
 
@@ -148,9 +151,13 @@ int main(int argc, char **argv)
     struct peer_entry * peer_table = calloc(501, sizeof (struct peer_entry));
     peer_table[500].pid = -1;
 
+    char * socket_path = getenv("LUAD_SOCKET_PATH");
+    if(! socket_path) {
+        error(1, 0, "Missing required environment variable LUAD_SOCKET_PATH");
+    }
+
     if(pipe(sigpipe_fds) < 0) {
-        perror("pipe");
-        exit(EXIT_FAILURE);
+        die("can't create pipe");
     }
     sigchild_fd = sigpipe_fds[1];
 
@@ -159,12 +166,11 @@ int main(int argc, char **argv)
     if (argc >= 2) {
         status = luaL_dofile(L, argv[1]);
         if (status) {
-            fprintf(stderr, "Error: %s\n", lua_tostring(L, -1));
-            return 1;
+            error(1, 0, "Error: %s", lua_tostring(L, -1));
         }
     }
 
-    int socket = open_socket("/tmp/mysock"); /* FIXME hardcoded */
+    int socket = open_socket(socket_path);
     struct sockaddr_un peer_addr;
     socklen_t peer_addr_len;
     char buf[1025];