Merge commit from fork

.changeset/breezy-moles-sin.md

@@ -0,0 +1,6 @@
+---
+'@backstage/backend-plugin-api': patch
+'@backstage/cli-common': patch
+---
+
+Move some of the symlink resolution to `isChildPath`

packages/backend-plugin-api/src/paths.test.ts

@@ -65,5 +65,85 @@ describe('paths', () => {
         `${workspacePath}/template`,
       );
     });
+
+    it('should throw an error if the path is a symlink pointing to a non-existent target outside base', () => {
+      // This tests the case where realpathSync would fail with ENOENT
+      // but the symlink itself exists and points outside the base directory
+      const nonExistentTarget = `${secondDirectory.path}/does-not-exist.txt`;
+      mockDir.addContent({
+        [`${workspacePath}/dangling-link`]: ({ symlink }) =>
+          symlink(nonExistentTarget),
+      });
+
+      expect(() =>
+        resolveSafeChildPath(workspacePath, './dangling-link'),
+      ).toThrow(
+        'Relative path is not allowed to refer to a directory outside its parent',
+      );
+    });
+
+    it('should allow symlinks pointing to non-existent targets within base directory', () => {
+      mockDir.addContent({
+        [`${workspacePath}/internal-link`]: ({ symlink }) =>
+          symlink('./future-file.txt'),
+      });
+
+      expect(resolveSafeChildPath(workspacePath, './internal-link')).toEqual(
+        `${workspacePath}/internal-link`,
+      );
+    });
+
+    it('should throw an error when writing through a symlink to a non-existent file outside base', () => {
+      // Symlink in workspace points outside, target directory exists but file doesn't
+      // e.g., /workspace/evil -> /etc, then write to evil/newfile.conf
+      // The check should catch that evil/newfile.conf resolves to /etc/newfile.conf
+      mockDir.addContent({
+        [`${workspacePath}/escape-link`]: ({ symlink }) =>
+          symlink(secondDirectory.path),
+      });
+
+      // This should throw because escape-link/new-file.txt would write to secondDirectory/new-file.txt
+      expect(() =>
+        resolveSafeChildPath(workspacePath, './escape-link/new-file.txt'),
+      ).toThrow(
+        'Relative path is not allowed to refer to a directory outside its parent',
+      );
+    });
+
+    it('should throw an error for symlink chains pointing outside base', () => {
+      // link1 -> link2 -> outside
+      // Even if final target doesn't exist, should detect the escape
+      const nonExistentOutside = `${secondDirectory.path}/does-not-exist`;
+      mockDir.addContent({
+        [`${workspacePath}/link2`]: ({ symlink }) =>
+          symlink(nonExistentOutside),
+        [`${workspacePath}/link1`]: ({ symlink }) => symlink('./link2'),
+      });
+
+      expect(() => resolveSafeChildPath(workspacePath, './link1')).toThrow(
+        'Relative path is not allowed to refer to a directory outside its parent',
+      );
+    });
+
+    it('should throw when deeply nested non-existent path has symlink ancestor pointing outside', () => {
+      // Tests the recursive parent-walking behavior in resolveRealPath.
+      // When given a/b/c/d/file.txt where none of b/c/d exist, the code walks up
+      // the tree until finding 'a' (a symlink), resolves it, then rebuilds the path.
+      mockDir.addContent({
+        [`${workspacePath}/escape`]: ({ symlink }) =>
+          symlink(secondDirectory.path),
+      });
+
+      // escape/deeply/nested/path/file.txt requires walking up 4 levels
+      // before finding the symlink at 'escape'
+      expect(() =>
+        resolveSafeChildPath(
+          workspacePath,
+          './escape/deeply/nested/path/file.txt',
+        ),
+      ).toThrow(
+        'Relative path is not allowed to refer to a directory outside its parent',
+      );
+    });
   });
 });

packages/backend-plugin-api/src/paths.ts

@@ -17,7 +17,6 @@
 import { isChildPath } from '@backstage/cli-common';
 import { NotAllowedError } from '@backstage/errors';
 import { resolve as resolvePath } from 'path';
-import { realpathSync as realPath } from 'fs';
 
 /** @internal */
 export const packagePathMocks = new Map<
@@ -63,10 +62,9 @@ export function resolvePackagePath(name: string, ...paths: string[]) {
  * @returns A path that is guaranteed to point to or within the base path.
  */
 export function resolveSafeChildPath(base: string, path: string): string {
-  const resolvedBasePath = resolveRealPath(base);
-  const targetPath = resolvePath(resolvedBasePath, path);
+  const targetPath = resolvePath(base, path);
 
-  if (!isChildPath(resolvedBasePath, resolveRealPath(targetPath))) {
+  if (!isChildPath(base, targetPath)) {
     throw new NotAllowedError(
       'Relative path is not allowed to refer to a directory outside its parent',
     );
@@ -76,16 +74,5 @@ export function resolveSafeChildPath(base: string, path: string): string {
   return resolvePath(base, path);
 }
 
-function resolveRealPath(path: string): string {
-  try {
-    return realPath(path);
-  } catch (ex) {
-    if (ex.code !== 'ENOENT') {
-      throw ex;
-    }
-  }
-
-  return path;
-}
 // Re-export isChildPath so that backend packages don't need to depend on cli-common
 export { isChildPath };

packages/cli-common/src/isChildPath.ts

@@ -14,7 +14,49 @@
  * limitations under the License.
  */
 
-import { relative, isAbsolute } from 'path';
+import {
+  relative,
+  isAbsolute,
+  resolve as resolvePath,
+  dirname,
+  basename,
+} from 'path';
+import { realpathSync, lstatSync, readlinkSync } from 'fs';
+
+// Resolves a path to its real location, following symlinks.
+// Handles cases where the final target doesn't exist by recursively
+// resolving parent directories.
+function resolveRealPath(path: string): string {
+  try {
+    return realpathSync(path);
+  } catch (ex) {
+    if (ex.code !== 'ENOENT') {
+      throw ex;
+    }
+  }
+
+  // Check if path itself is a dangling symlink - recursively resolve the target
+  // to handle symlink chains (e.g., link1 -> link2 -> /outside)
+  try {
+    if (lstatSync(path).isSymbolicLink()) {
+      const target = resolvePath(dirname(path), readlinkSync(path));
+      return resolveRealPath(target);
+    }
+  } catch (ex) {
+    if (ex.code !== 'ENOENT') {
+      throw ex;
+    }
+  }
+
+  // Path doesn't exist - walk up the tree until we find an existing path,
+  // resolve it, then rebuild the non-existent portion on top
+  const parent = dirname(path);
+  if (parent === path) {
+    return path; // Hit filesystem root
+  }
+
+  return resolvePath(resolveRealPath(parent), basename(path));
+}
 
 /**
  * Checks if path is the same as or a child path of base.
@@ -22,7 +64,10 @@ import { relative, isAbsolute } from 'path';
  * @public
  */
 export function isChildPath(base: string, path: string): boolean {
-  const relativePath = relative(base, path);
+  const resolvedBase = resolveRealPath(base);
+  const resolvedPath = resolveRealPath(path);
+
+  const relativePath = relative(resolvedBase, resolvedPath);
   if (relativePath === '') {
     // The same directory
     return true;