fix(ln): allow symlinks in ReadWrite RealFs mounts (#1251)

Closes #1158

Author: chaliy

crates/bashkit/src/fs/posix.rs

@@ -242,9 +242,10 @@ impl<B: FsBackend + 'static> FileSystem for PosixFs<B> {
     }
 
     async fn symlink(&self, target: &Path, link: &Path) -> Result<()> {
-        let target = Self::normalize(target);
+        // Don't normalize target: symlink targets are stored as-is on disk.
+        // Normalizing a relative target to absolute would break containment checks.
         let link = Self::normalize(link);
-        self.backend.symlink(&target, &link).await
+        self.backend.symlink(target, &link).await
     }
 
     async fn read_link(&self, path: &Path) -> Result<PathBuf> {

crates/bashkit/src/fs/realfs.rs

@@ -414,16 +414,46 @@ impl FsBackend for RealFs {
         Ok(())
     }
 
-    /// THREAT[TM-ESC-003]: Symlink creation is blocked in RealFs to prevent
-    /// sandbox escape. Even though bashkit itself doesn't follow symlinks
-    /// (TM-ESC-002), any external process sharing the directory tree would
-    /// follow them, enabling reads/writes to arbitrary host paths.
-    async fn symlink(&self, _target: &Path, _link: &Path) -> Result<()> {
-        Err(IoError::new(
-            ErrorKind::PermissionDenied,
-            "symlink creation is not allowed in RealFs (sandbox security)",
-        )
-        .into())
+    /// THREAT[TM-ESC-003]: Symlink creation in RealFs is allowed only in
+    /// ReadWrite mode. The OS resolves symlink targets on the host filesystem,
+    /// so we must validate that the effective target stays within the mount
+    /// root on disk. Absolute targets are rejected. Relative targets are
+    /// normalized against the link's host-side parent directory.
+    async fn symlink(&self, target: &Path, link: &Path) -> Result<()> {
+        self.check_writable()?;
+        let real_link = self.resolve(link)?;
+
+        // Absolute targets always escape the mount root on disk
+        if target.is_absolute() {
+            return Err(IoError::new(
+                ErrorKind::PermissionDenied,
+                "symlink with absolute target not allowed in RealFs (sandbox security)",
+            )
+            .into());
+        }
+
+        // Relative targets: resolve against the link's host-side parent
+        // to verify the effective path stays within root
+        let link_parent = real_link.parent().unwrap_or(&self.root);
+        let effective = normalize_host_path(&link_parent.join(target));
+        if !effective.starts_with(&self.root) {
+            return Err(IoError::new(
+                ErrorKind::PermissionDenied,
+                "symlink target escapes realfs root (sandbox security)",
+            )
+            .into());
+        }
+
+        #[cfg(unix)]
+        {
+            tokio::fs::symlink(target, &real_link).await?;
+        }
+        #[cfg(not(unix))]
+        {
+            let _ = target;
+            tokio::fs::write(&real_link, "").await?;
+        }
+        Ok(())
     }
 
     async fn read_link(&self, path: &Path) -> Result<PathBuf> {

crates/bashkit/tests/realfs_tests.rs

@@ -343,6 +343,28 @@ async fn realfs_symlink_relative_escape_blocked() {
     );
 }
 
+#[tokio::test]
+async fn realfs_symlink_within_mount_allowed() {
+    let dir = setup_host_dir();
+    std::fs::write(dir.path().join("original.txt"), "content").unwrap();
+
+    let mut bash = Bash::builder()
+        .mount_real_readwrite_at(dir.path(), "/mnt/workspace")
+        .build();
+
+    // Relative symlink within mount should succeed (exit code 0)
+    let r = bash
+        .exec("ln -s original.txt /mnt/workspace/link.txt 2>&1; echo $?")
+        .await
+        .unwrap();
+    assert!(
+        r.stdout.trim().ends_with('0'),
+        "Symlink within mount should succeed, got stdout: {} stderr: {}",
+        r.stdout,
+        r.stderr
+    );
+}
+
 // --- Runtime mount/unmount (exercises Bash::mount / Bash::unmount) ---
 
 #[tokio::test]