fix: harden the per-user IPC endpoint

ipc.c

@@ -1,7 +1,10 @@
 #include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <sys/socket.h>
+#include <sys/un.h>
 #include "ipc.h"
 
 #if !defined(MSG_NOSIGNAL) && !defined(SO_NOSIGPIPE)
@@ -21,6 +24,49 @@ getlen(const unsigned char p[Ipclensz])
 	return p[0] | (p[1] << 8);
 }
 
+int
+ipcpath(char *dst, size_t cap)
+{
+	const char *dir, *sep;
+	int n;
+
+	if(dst == NULL || cap == 0)
+		return -1;
+	dir = getenv("XDG_RUNTIME_DIR");
+	if(dir != NULL && dir[0] == '/'){
+		sep = dir[strlen(dir)-1] == '/' ? "" : "/";
+		n = snprintf(dst, cap, "%s%sstrans.sock", dir, sep);
+	}else
+		n = snprintf(dst, cap, "/tmp/strans.%lu",
+			(unsigned long)getuid());
+	if(n < 0 || (size_t)n >= cap)
+		return -1;
+	return 0;
+}
+
+int
+ipcconnect(void)
+{
+	struct sockaddr_un addr;
+	int e, fd;
+
+	memset(&addr, 0, sizeof addr);
+	addr.sun_family = AF_UNIX;
+	if(ipcpath(addr.sun_path, sizeof addr.sun_path) < 0){
+		errno = ENAMETOOLONG;
+		return -1;
+	}
+	fd = socket(AF_UNIX, SOCK_STREAM, 0);
+	if(fd < 0)
+		return -1;
+	if(connect(fd, (struct sockaddr*)&addr, sizeof addr) == 0)
+		return fd;
+	e = errno;
+	close(fd);
+	errno = e;
+	return -1;
+}
+
 void
 ipcpackreq(unsigned char req[Ipcreqsz], int want, unsigned int mod,
 	unsigned int key)