path/filepath: add IsLocal

IsLocal reports whether a path lexically refers to a location
contained within the directory in which it is evaluated.
It identifies paths that are absolute, escape a directory
with ".." elements, and (on Windows) paths that reference
reserved device names.

For #56219.

Change-Id: I35edfa3ce77b40b8e66f1fc8e0ff73cfd06f2313
Reviewed-on: https://go-review.googlesource.com/c/go/+/449239
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>

1 files changed, 40 insertions, 0 deletions

diff --git a/src/path/filepath/path.go b/src/path/filepath/path.go
index c5c54fc9a5..a6578cbb72 100644
--- a/src/path/filepath/path.go
+++ b/src/path/filepath/path.go
@@ -172,6 +172,46 @@ func Clean(path string) string {
 	return FromSlash(out.string())
 }
 
+// IsLocal reports whether path, using lexical analysis only, has all of these properties:
+//
+//   - is within the subtree rooted at the directory in which path is evaluated
+//   - is not an absolute path
+//   - is not empty
+//   - on Windows, is not a reserved name such as "NUL"
+//
+// If IsLocal(path) returns true, then
+// Join(base, path) will always produce a path contained within base and
+// Clean(path) will always produce an unrooted path with no ".." path elements.
+//
+// IsLocal is a purely lexical operation.
+// In particular, it does not account for the effect of any symbolic links
+// that may exist in the filesystem.
+func IsLocal(path string) bool {
+	return isLocal(path)
+}
+
+func unixIsLocal(path string) bool {
+	if IsAbs(path) || path == "" {
+		return false
+	}
+	hasDots := false
+	for p := path; p != ""; {
+		var part string
+		part, p, _ = strings.Cut(p, "/")
+		if part == "." || part == ".." {
+			hasDots = true
+			break
+		}
+	}
+	if hasDots {
+		path = Clean(path)
+	}
+	if path == ".." || strings.HasPrefix(path, "../") {
+		return false
+	}
+	return true
+}
+
 // ToSlash returns the result of replacing each separator character
 // in path with a slash ('/') character. Multiple separators are
 // replaced by multiple slashes.