diff --git a/source3/lib/util_path.c b/source3/lib/util_path.c
index c34b734384c..e6bed724551 100644
--- a/source3/lib/util_path.c
+++ b/source3/lib/util_path.c
@@ -23,6 +23,8 @@
 
 #include "replace.h"
 #include <talloc.h>
+#include "lib/util/debug.h"
+#include "lib/util/fault.h"
 #include "lib/util/samba_util.h"
 #include "lib/util_path.h"
 
@@ -210,3 +212,53 @@ char *canonicalize_absolute_path(TALLOC_CTX *ctx, const char *pathname_in)
 	*p++ = '\0';
 	return pathname;
 }
+
+/*
+ * Take two absolute paths, figure out if "subdir" is a proper
+ * subdirectory of "parent". Return the component relative to the
+ * "parent" without the potential "/". Take care of "parent"
+ * possibly ending in "/".
+ */
+bool subdir_of(const char *parent,
+	       size_t parent_len,
+	       const char *subdir,
+	       const char **_relative)
+{
+	const char *relative = NULL;
+	bool matched;
+
+	SMB_ASSERT(parent[0] == '/');
+	SMB_ASSERT(subdir[0] == '/');
+
+	if (parent_len == 1) {
+		/*
+		 * Everything is below "/"
+		 */
+		*_relative = subdir+1;
+		return true;
+	}
+
+	if (parent[parent_len-1] == '/') {
+		parent_len -= 1;
+	}
+
+	matched = (strncmp(subdir, parent, parent_len) == 0);
+	if (!matched) {
+		return false;
+	}
+
+	relative = &subdir[parent_len];
+
+	if (relative[0] == '\0') {
+		*_relative = relative; /* nothing left */
+		return true;
+	}
+
+	if (relative[0] == '/') {
+		/* End of parent must match a '/' in subdir. */
+		*_relative = relative+1;
+		return true;
+	}
+
+	return false;
+}
diff --git a/source3/lib/util_path.h b/source3/lib/util_path.h
index 3e7d04de550..0ea508bf5bb 100644
--- a/source3/lib/util_path.h
+++ b/source3/lib/util_path.h
@@ -31,5 +31,9 @@ char *lock_path(TALLOC_CTX *mem_ctx, const char *name);
 char *state_path(TALLOC_CTX *mem_ctx, const char *name);
 char *cache_path(TALLOC_CTX *mem_ctx, const char *name);
 char *canonicalize_absolute_path(TALLOC_CTX *ctx, const char *abs_path);
+bool subdir_of(const char *parent,
+	       size_t parent_len,
+	       const char *subdir,
+	       const char **_relative);
 
 #endif