Custom comparator for paths in FileAccessTree

Author: prdoyle

libs/entitlement/src/main/java/org/elasticsearch/entitlement/runtime/policy/FileAccessTree.java

@@ -14,6 +14,7 @@
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Comparator;
 import java.util.List;
 import java.util.Objects;
 
@@ -46,8 +47,8 @@ private FileAccessTree(FilesEntitlement filesEntitlement, PathLookup pathLookup)
         readPaths.add(tempDir);
         writePaths.add(tempDir);
 
-        readPaths.sort(String::compareTo);
-        writePaths.sort(String::compareTo);
+        readPaths.sort(PATH_ORDER);
+        writePaths.sort(PATH_ORDER);
 
         this.readPaths = pruneSortedPaths(readPaths).toArray(new String[0]);
         this.writePaths = pruneSortedPaths(writePaths).toArray(new String[0]);
@@ -60,7 +61,7 @@ private static List<String> pruneSortedPaths(List<String> paths) {
             prunedReadPaths.add(currentPath);
             for (int i = 1; i < paths.size(); ++i) {
                 String nextPath = paths.get(i);
-                if (nextPath.startsWith(currentPath) == false) {
+                if (isParent(currentPath, nextPath) == false) {
                     prunedReadPaths.add(nextPath);
                     currentPath = nextPath;
                 }
@@ -95,14 +96,17 @@ private static boolean checkPath(String path, String[] paths) {
         if (paths.length == 0) {
             return false;
         }
-        int ndx = Arrays.binarySearch(paths, path);
+        int ndx = Arrays.binarySearch(paths, path, PATH_ORDER);
         if (ndx < -1) {
-            String maybeParent = paths[-ndx - 2];
-            return path.startsWith(maybeParent) && path.startsWith(FILE_SEPARATOR, maybeParent.length());
+            return isParent(paths[-ndx - 2], path);
         }
         return ndx >= 0;
     }
 
+    private static boolean isParent(String maybeParent, String path) {
+        return path.startsWith(maybeParent) && path.startsWith(FILE_SEPARATOR, maybeParent.length());
+    }
+
     @Override
     public boolean equals(Object o) {
         if (o == null || getClass() != o.getClass()) return false;
@@ -114,4 +118,30 @@ public boolean equals(Object o) {
     public int hashCode() {
         return Objects.hash(Arrays.hashCode(readPaths), Arrays.hashCode(writePaths));
     }
+
+    /**
+     * For our lexicographic sort trick to work correctly, we must have path separators sort before
+     * any other character so that files in a directory appear immediately after that directory.
+     * For example, we require [/a, /a/b, /a.xml] rather than the natural order [/a, /a.xml, /a/b].
+     */
+    private static final Comparator<String> PATH_ORDER = (s1, s2) -> {
+        Path p1 = Path.of(s1);
+        Path p2 = Path.of(s2);
+        var i1 = p1.iterator();
+        var i2 = p2.iterator();
+        while (i1.hasNext() && i2.hasNext()) {
+            int cmp = i1.next().compareTo(i2.next());
+            if (cmp != 0) {
+                return cmp;
+            }
+        }
+        if (i1.hasNext()) {
+            return 1;
+        } else if (i2.hasNext()) {
+            return -1;
+        } else {
+            assert p1.equals(p2);
+            return 0;
+        }
+    };
 }

libs/entitlement/src/test/java/org/elasticsearch/entitlement/runtime/policy/FileAccessTreeTests.java

@@ -117,6 +117,15 @@ public void testPrunedPaths() {
         assertThat(tree.canWrite(path("foo/baz")), is(false));
     }
 
+    public void testPathAndFileWithSamePrefix() {
+        var tree = accessTree(entitlement("foo/bar/", "read", "foo/bar.xml", "read"));
+        assertThat(tree.canRead(path("foo")), is(false));
+        assertThat(tree.canRead(path("foo/bar")), is(true));
+        assertThat(tree.canRead(path("foo/bar/baz")), is(true));
+        assertThat(tree.canRead(path("foo/bar.xml")), is(true));
+        assertThat(tree.canRead(path("foo/bar.txt")), is(false));
+    }
+
     public void testReadWithRelativePath() {
         for (var dir : List.of("config", "home")) {
             var tree = accessTree(entitlement(Map.of("relative_path", "foo", "mode", "read", "relative_to", dir)));