add a bunch of bounds checking to pytree

It's possible to pass arbitrary string input to pytree from Python; let's not have a bunch of low-hanging memory safety issues.

Differential Revision: [D68166303](https://our.internmc.facebook.com/intern/diff/D68166303/)

[ghstack-poisoned]

Author: Github Executorch

extension/pytree/pytree.h

@@ -60,7 +60,7 @@ struct Key {
   std::variant<std::monostate, KeyInt, KeyStr> repr_;
 
  public:
-  Key() {}
+  Key() = default;
   /*implicit*/ Key(KeyInt key) : repr_(key) {}
   /*implicit*/ Key(KeyStr key) : repr_(std::move(key)) {}
 
@@ -131,7 +131,7 @@ struct ContainerHandle {
   using leaf_type = T;
   std::unique_ptr<container_type> handle;
 
-  ContainerHandle() {}
+  ContainerHandle() = default;
 
   template <typename... Args>
   ContainerHandle(Args... args)
@@ -427,6 +427,20 @@ struct arr {
     return data_[idx];
   }
 
+  T& at(size_t idx) {
+    if (idx >= size()) {
+      throw std::out_of_range("bounds check failed in pytree arr");
+    }
+    return data_[idx];
+  }
+
+  const T& at(size_t idx) const {
+    if (idx >= size()) {
+      throw std::out_of_range("bounds check failed in pytree arr");
+    }
+    return data_[idx];
+  }
+
   inline T* data() {
     return data_.get();
   }
@@ -458,7 +472,7 @@ struct arr {
 
 inline size_t read_number(const StrTreeSpec& spec, size_t& read_idx) {
   size_t num = 0;
-  while (isdigit(spec[read_idx])) {
+  while (isdigit(spec.at(read_idx))) {
     num = 10 * num + (spec[read_idx] - '0');
     read_idx++;
   }
@@ -470,19 +484,22 @@ inline arr<size_t> read_node_layout(const StrTreeSpec& spec, size_t& read_idx) {
   arr<size_t> ret(child_num);
 
   size_t child_idx = 0;
-  while (spec[read_idx] == Config::kChildrenDataSep) {
+  while (spec.at(read_idx) == Config::kChildrenDataSep) {
     ++read_idx;
-    ret[child_idx++] = read_number(spec, read_idx);
+    ret.at(child_idx++) = read_number(spec, read_idx);
   }
   return ret;
 }
 
+// spec_data comes from pre_parse, which guarantees 1)
+// spec_data.size() == spec.size() and 2) contents of spec_data are
+// in-bounds indices for spec, so we omit bounds checks for spec_data.
 template <typename Aux>
 TreeSpec<Aux> from_str_internal(
     const StrTreeSpec& spec,
     size_t read_idx,
     const arr<size_t>& spec_data) {
-  const auto kind_char = spec[read_idx];
+  const auto kind_char = spec.at(read_idx);
   switch (kind_char) {
     case Config::kTuple:
     case Config::kNamedTuple:
@@ -496,7 +513,7 @@ TreeSpec<Aux> from_str_internal(
       } else if (Config::kCustom == kind_char) {
         kind = Kind::Custom;
         read_idx++;
-        assert(spec[read_idx] == '(');
+        assert(spec.at(read_idx) == '(');
         auto type_str_end = spec_data[read_idx];
         read_idx++;
         custom_type = spec.substr(read_idx, type_str_end - read_idx);
@@ -515,10 +532,14 @@ TreeSpec<Aux> from_str_internal(
       size_t leaves_offset = 0;
 
       if (size > 0) {
-        while (spec[read_idx] != Config::kNodeDataEnd) {
+        while (spec.at(read_idx) != Config::kNodeDataEnd) {
           // NOLINTNEXTLINE
           auto next_delim_idx = spec_data[read_idx];
           read_idx++;
+          if (child_idx >= size) {
+            throw std::out_of_range(
+                "bounds check failed writing to pytree item");
+          }
           c->items[child_idx] =
               from_str_internal<Aux>(spec, read_idx, spec_data);
           read_idx = next_delim_idx;
@@ -541,11 +562,14 @@ TreeSpec<Aux> from_str_internal(
       size_t leaves_offset = 0;
 
       if (size > 0) {
-        while (spec[read_idx] != Config::kNodeDataEnd) {
+        while (spec.at(read_idx) != Config::kNodeDataEnd) {
           // NOLINTNEXTLINE
           auto next_delim_idx = spec_data[read_idx];
           read_idx++;
-          if (spec[read_idx] == Config::kDictStrKeyQuote) {
+          if (child_idx >= size) {
+            throw std::out_of_range("bounds check failed decoding pytree dict");
+          }
+          if (spec.at(read_idx) == Config::kDictStrKeyQuote) {
             auto key_delim_idx = spec_data[read_idx];
             read_idx++;
             const size_t key_len = key_delim_idx - read_idx;
@@ -562,7 +586,7 @@ TreeSpec<Aux> from_str_internal(
           c->items[child_idx] =
               from_str_internal<Aux>(spec, read_idx, spec_data);
           read_idx = next_delim_idx;
-          leaves_offset += layout[child_idx++];
+          leaves_offset += layout.at(child_idx++);
         }
       } else {
         read_idx++;
@@ -605,7 +629,9 @@ struct stack final {
   }
 };
 
+// We guarantee indicies in the result are in bounds.
 inline arr<size_t> pre_parse(const StrTreeSpec& spec) {
+  // Invariant: indices in stack are in bounds.
   stack<std::pair<size_t, size_t>> stack;
   size_t i = 0;
   const size_t size = spec.size();
@@ -627,11 +653,15 @@ inline arr<size_t> pre_parse(const StrTreeSpec& spec) {
       case Config::kDictStrKeyQuote: {
         size_t idx = i;
         i++;
-        while (spec[i] != Config::kDictStrKeyQuote) {
+        while (spec.at(i) != Config::kDictStrKeyQuote) {
           i++;
         }
-        ret[idx] = i;
-        ret[i] = idx;
+        if (i >= size) {
+          throw std::out_of_range(
+              "bounds check failed while parsing dictionary key");
+        }
+        ret.at(idx) = i;
+        ret.at(i) = idx;
         break;
       }
       case Config::kChildrenSep: {

extension/pytree/test/test_pytree.cpp

@@ -183,3 +183,22 @@ TEST(pytree, FlattenNestedDict) {
     ASSERT_EQ(*leaves[i], items[i]);
   }
 }
+
+TEST(pytree, EmptySpec) {
+  Leaf items[1] = {9};
+  EXPECT_THROW(unflatten("", items), std::out_of_range);
+}
+
+TEST(pytree, BoundsCheckListLayout) {
+  // Malformed: layout one child, have two
+  std::string spec = "L1#1($,$)";
+  Leaf items[2] = {11, 12};
+  EXPECT_THROW(unflatten(spec, items), std::out_of_range);
+}
+
+TEST(pytree, BoundsCheckDictLayout) {
+  // Malformed: layout one child, have two.
+  std::string spec = "D1#1('key0':$,'key1':$)";
+  Leaf items[2] = {11, 12};
+  EXPECT_THROW(unflatten(spec, items), std::out_of_range);
+}