closures: Create temporary file with O_TMPFILE and O_CLOEXEC when ava…

…ilable

The open_temp_exec_file_dir function can create a temporary file without
file system accessible link. If the O_TMPFILE flag is not defined (old
Linux kernel or libc) the behavior is unchanged.

The open_temp_exec_file_name function now need a new argument "flags"
(like O_CLOEXEC) used for temporary file creation.

The O_TMPFILE flag allow temporary file creation without race condition.
This feature/fix prevent another process to access the (future)
executable file from the file system.

The O_CLOEXEC flag automatically close the temporary file for any
execve. This avoid transmitting (executable) file descriptor to a child
process.

src/closures.c

@@ -265,9 +265,9 @@ static size_t execsize = 0;
 
 /* Open a temporary file name, and immediately unlink it.  */
 static int
-open_temp_exec_file_name (char *name)
+open_temp_exec_file_name (char *name, int flags)
 {
-  int fd = mkstemp (name);
+  int fd = mkostemp (name, flags);
 
   if (fd != -1)
     unlink (name);
@@ -280,16 +280,35 @@ static int
 open_temp_exec_file_dir (const char *dir)
 {
   static const char suffix[] = "/ffiXXXXXX";
-  size_t lendir = strlen (dir);
-  char *tempname = __builtin_alloca (lendir + sizeof (suffix));
+  int lendir, flags, fd;
+  char *tempname;
+
+#ifdef O_CLOEXEC
+  flags = O_CLOEXEC;
+#else
+  flags = 0;
+#endif
+
+#ifdef O_TMPFILE
+  fd = open (dir, flags | O_RDWR | O_EXCL | O_TMPFILE, 0700);
+  /* If the running system does not support the O_TMPFILE flag then retry without it. */
+  if (fd != -1 || (errno != EINVAL && errno != EISDIR && errno != EOPNOTSUPP)) {
+    return fd;
+  } else {
+    errno = 0;
+  }
+#endif
+
+  lendir = strlen (dir);
+  tempname = __builtin_alloca (lendir + sizeof (suffix));
 
   if (!tempname)
     return -1;
 
   memcpy (tempname, dir, lendir);
   memcpy (tempname + lendir, suffix, sizeof (suffix));
 
-  return open_temp_exec_file_name (tempname);
+  return open_temp_exec_file_name (tempname, flags);
 }
 
 /* Open a temporary file in the directory in the named environment