utils: Add flatpak_parse_fd

This is meant to parse file descriptor strings passed via the command
line. It is not a security mechanism and will happily accept fds 0-3 as
well.

common/flatpak-context.c

@@ -2432,21 +2432,16 @@ option_env_fd_cb (const gchar *option_name,
                   GError     **error)
 {
   FlatpakContext *context = data;
-  guint64 fd;
-  gchar *endptr;
-  gboolean ret;
+  glnx_autofd int fd = -1;
 
-  fd = g_ascii_strtoull (value, &endptr, 10);
+  fd = flatpak_parse_fd (value, error);
+  if (fd < 0)
+    return FALSE;
 
-  if (endptr == NULL || *endptr != '\0' || fd > G_MAXINT)
-    return glnx_throw (error, "Not a valid file descriptor: %s", value);
+  if (fd < 3)
+    return glnx_throw (error, "File descriptors 0, 1, 2 are reserved");
 
-  ret = flatpak_context_parse_env_fd (context, (int) fd, error);
-
-  if (fd >= 3)
-    close (fd);
-
-  return ret;
+  return flatpak_context_parse_env_fd (context, fd, error);
 }
 
 static gboolean

common/flatpak-utils-private.h

@@ -365,6 +365,9 @@ gboolean running_under_sudo_root (void);
 void flatpak_set_debugging (gboolean debugging);
 gboolean flatpak_is_debugging (void);
 
+int flatpak_parse_fd (const char  *fd_string,
+                      GError     **error);
+
 #ifdef INCLUDE_INTERNAL_TESTS
 typedef void (*flatpak_test_fn) (void);
 void flatpak_add_test (const char *path, flatpak_test_fn fn);

common/flatpak-utils.c

@@ -2582,6 +2582,28 @@ flatpak_is_debugging (void)
   return is_debugging;
 }
 
+int
+flatpak_parse_fd (const char  *fd_string,
+                  GError     **error)
+{
+  guint64 parsed;
+  char *endptr;
+  int fd;
+  struct stat stbuf;
+
+  parsed = g_ascii_strtoull (fd_string, &endptr, 10);
+
+  if (endptr == NULL || *endptr != '\0' || parsed > G_MAXINT)
+    return glnx_fd_throw (error, "Not a valid file descriptor: %s", fd_string);
+
+  fd = (int) parsed;
+
+  if (!glnx_fstat (fd, &stbuf, error))
+    return -1;
+
+  return fd;
+}
+
 #ifdef INCLUDE_INTERNAL_TESTS
 static GList *flatpak_test_paths = NULL;
 static GList *flatpak_test_fns = NULL;