Allow verification of buffers starting from specified root

Author: Muon

rust/flatbuffers/src/verifier.rs

@@ -275,7 +275,7 @@ fn trace_elem<T>(res: Result<T>, index: usize, position: usize) -> Result<T> {
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
-pub struct VerifierOptions {
+pub struct VerifierOptions<'a> {
     /// Maximum depth of nested tables allowed in a valid flatbuffer.
     pub max_depth: usize,
     /// Maximum number of tables allowed in a valid flatbuffer.
@@ -289,16 +289,20 @@ pub struct VerifierOptions {
     // probably want an option to ignore utf8 errors since strings come from c++
     // options to error un-recognized enums and unions? possible footgun.
     // Ignore nested flatbuffers, etc?
+
+    /// The name of the table to use as the root table instead of the schema root.
+    pub root_table_name: Option<&'a str>,
 }
 
-impl Default for VerifierOptions {
+impl Default for VerifierOptions<'_> {
     fn default() -> Self {
         Self {
             max_depth: 64,
             max_tables: 1_000_000,
             // size_ might do something different.
             max_apparent_size: 1 << 31,
             ignore_missing_null_terminator: false,
+            root_table_name: None,
         }
     }
 }
@@ -307,7 +311,7 @@ impl Default for VerifierOptions {
 #[derive(Debug)]
 pub struct Verifier<'opts, 'buf> {
     buffer: &'buf [u8],
-    opts: &'opts VerifierOptions,
+    opts: &'opts VerifierOptions<'opts>,
     depth: usize,
     num_tables: usize,
     apparent_size: usize,

rust/reflection/src/reflection_verifier.rs

@@ -30,8 +30,14 @@ pub fn verify_with_options(
     buf_loc_to_obj_idx: &mut HashMap<usize, i32>,
 ) -> FlatbufferResult<()> {
     let mut verifier = Verifier::new(opts, buffer);
-    if let Some(table_object) = schema.root_table() {
-        if let core::result::Result::Ok(table_pos) = verifier.get_uoffset(0) {
+    let root_table = match opts.root_table_name {
+        Some(root_table_name) => schema
+            .objects()
+            .lookup_by_key(root_table_name, |o, k| o.key_compare_with_value(k)),
+        None => schema.root_table(),
+    };
+    if let Some(table_object) = root_table {
+        if let Ok(table_pos) = verifier.get_uoffset(0) {
             // Inserts -1 as object index for root table
             buf_loc_to_obj_idx.insert(table_pos.try_into()?, -1);
             let mut verified = vec![false; buffer.len()];