Simplify coding around path_contains_parent_reference().

Given the existing stipulation that path_contains_parent_reference()
must only be invoked on canonicalized paths, we can simplify things
in the wake of commit c10f830c5.  It is now only possible to see
".." at the start of a relative path.  That means we can simplify
path_contains_parent_reference() itself quite a bit, and it makes
the two existing outside call sites dead code, since they'd already
checked that the path is absolute.

We could now fold path_contains_parent_reference() into its only
remaining caller path_is_relative_and_below_cwd().  But it seems
better to leave it as a separately callable function, in case any
extensions are using it.

Also document the pre-existing requirement for
path_is_relative_and_below_cwd's input to be likewise canonicalized.

Shenhao Wang and Tom Lane

Discussion: https://postgr.es/m/OSBPR01MB4214FA221FFE046F11F2AD74F2D49@OSBPR01MB4214.jpnprd01.prod.outlook.com

contrib/adminpack/adminpack.c

@@ -88,12 +88,6 @@ convert_and_check_filename(text *arg)
 	 */
 	if (is_absolute_path(filename))
 	{
-		/* Disallow '/a/b/data/..' */
-		if (path_contains_parent_reference(filename))
-			ereport(ERROR,
-					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-					 errmsg("reference to parent directory (\"..\") not allowed")));
-
 		/* Allow absolute paths if within DataDir */
 		if (!path_is_prefix_of_path(DataDir, filename))
 			ereport(ERROR,

src/backend/utils/adt/genfile.c

@@ -72,12 +72,6 @@ convert_and_check_filename(text *arg)
 	 */
 	if (is_absolute_path(filename))
 	{
-		/* Disallow '/a/b/data/..' */
-		if (path_contains_parent_reference(filename))
-			ereport(ERROR,
-					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-					 errmsg("reference to parent directory (\"..\") not allowed")));
-
 		/*
 		 * Allow absolute paths if within DataDir or Log_directory, even
 		 * though Log_directory might be outside DataDir.

src/port/path.c

@@ -494,27 +494,21 @@ canonicalize_path(char *path)
  * Detect whether a path contains any parent-directory references ("..")
  *
  * The input *must* have been put through canonicalize_path previously.
- *
- * This is a bit tricky because we mustn't be fooled by "..a.." (legal)
- * nor "C:.." (legal on Unix but not Windows).
  */
 bool
 path_contains_parent_reference(const char *path)
 {
-	int			path_len;
+	/*
-
+	 * Once canonicalized, an absolute path cannot contain any ".." at all,
+	 * while a relative path could contain ".."(s) only at the start.  So it
+	 * is sufficient to check the start of the path, after skipping any
+	 * Windows drive/network specifier.
+	 */
 	path = skip_drive(path);	/* C: shouldn't affect our conclusion */
 
-	path_len = strlen(path);
+	if (path[0] == '.' &&
-
+		path[1] == '.' &&
-	/*
+		(path[2] == '\0' || path[2] == '/'))
-	 * ".." could be the whole path; otherwise, if it's present it must be at
-	 * the beginning, in the middle, or at the end.
-	 */
-	if (strcmp(path, "..") == 0 ||
-		strncmp(path, "../", 3) == 0 ||
-		strstr(path, "/../") != NULL ||
-		(path_len >= 3 && strcmp(path + path_len - 3, "/..") == 0))
 		return true;
 
 	return false;
@@ -522,10 +516,11 @@ path_contains_parent_reference(const char *path)
 
 /*
  * Detect whether a path is only in or below the current working directory.
+ *
+ * The input *must* have been put through canonicalize_path previously.
+ *
  * An absolute path that matches the current working directory should
- * return false (we only want relative to the cwd).  We don't allow
+ * return false (we only want relative to the cwd).
- * "/../" even if that would keep us under the cwd (it is too hard to
- * track that).
  */
 bool
 path_is_relative_and_below_cwd(const char *path)