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Read UBJSON and BJData containers without recursing per nesting level

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get_ubjson_array() and get_ubjson_object() read their elements by calling
back into the value reader, which called them again for a nested container,
so the native call stack grew with the nesting depth of the input. '[' alone
opens a container, so half a million of them crashes the process before the
input runs out (#5104). The optimized forms reach the same path through a
size or type annotation, and in plain UBJSON '[' and '{' are permitted as the
type of an optimized container, so "[$[#i\x01" repeated nests just as deeply
at six bytes a level.

Both readers now only open their container, and parse_ubjson_internal() loops:
it closes the containers that have ended, claims the next element of the
innermost one, reads its key when it is an object, and works out the marker
of the value to read next. That last part is where the formats differ, and
the loop follows what the four element loops used to do:

  - a sized, typed container gives its elements no marker of their own
  - a sized, untyped container reads one for each element
  - a container that ends at a marker has the byte already, from the test
    against ']' or '}'; for an object it is the first byte of the key

The ND-array wrapper and the 'B' binary shortcut stay as they are. Both read
a complete value rather than opening a container, and their elements are
always scalars: BJData does not permit '[' or '{' as an optimized type, which
is also why only plain UBJSON needed the type-marker case above.

A container of no-ops keeps its behaviour of holding no elements while still
announcing its declared size to the SAX parser, by opening it and then
setting its count to zero.

unit-ubjson and unit-bjdata pass unchanged, 1.39 million assertions between
them, and a behaviour comparison against the previous commit over every
container form -- sized, unsized, typed, untyped, empty, no-op, ND-array,
binary, and the forms nested inside one another -- gives identical values,
error codes, messages and byte offsets. 500,000 levels of each vector now
report a parse error instead of crashing, and a well-formed 100,000-level
value is read to completion.

The driver costs about 3 % on parsing 60,000 small objects and one array of a
million integers, for the reason given in the previous commit; reading the
frame once per element rather than per branch halved what it cost before.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-06 18:29:42 +02:00
parent 68e4094cdc
commit 7dc308d952
3 changed files with 325 additions and 198 deletions
Download Patch File Download Diff File Expand all files Collapse all files
single_include/nlohmann/json.hpp
+110 -99
View File
@@ -10886,8 +10886,12 @@ class binary_reader
*/
struct container_frame
{
/// number of elements that have not been read yet
/// number of elements that have not been read yet, or npos when the
/// container is not sized and ends at a marker instead
std::size_t remaining = 0;
/// UBJSON/BJData: the type marker of an optimized container, so that
/// its elements are read without one of their own; 0 otherwise
char_int_type type_marker = 0;
/// whether to close this container with end_object() or end_array()
bool is_object = false;
};
@@ -10904,7 +10908,8 @@ class binary_reader
@return whether the SAX parser accepted the start event
*/
bool enter_container(const bool is_object, const std::size_t len)
bool enter_container(const bool is_object, const std::size_t len,
const char_int_type type_marker = 0)
{
if (JSON_HEDLEY_UNLIKELY(is_object ? !sax->start_object(len) : !sax->start_array(len)))
{
@@ -10913,6 +10918,7 @@ class binary_reader
container_frame frame;
frame.remaining = len;
frame.type_marker = type_marker;
frame.is_object = is_object;
container_stack.push_back(frame);
return true;
@@ -12845,7 +12851,99 @@ class binary_reader
*/
bool parse_ubjson_internal(const bool get_char = true)
{
return get_ubjson_value(get_char ? get_ignore_noop() : current);
// the key currently being read; hoisted out of the loop so that its
// capacity is reused across elements and across nesting levels
string_t key;
// the type marker of the value to read next
char_int_type prefix = get_char ? get_ignore_noop() : current;
while (true)
{
const std::size_t depth = container_stack.size();
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_value(prefix)))
{
return false;
}
// the value begun here is complete once it is not inside anything
if (container_stack.empty())
{
return true;
}
// a value was completed rather than a container opened; a
// container that ends at a marker needs the next byte to test
if (container_stack.size() == depth && container_stack.back().remaining == npos)
{
get_ignore_noop();
}
// advance to the next element, closing the containers that ended.
// The reference is not held across get_ubjson_value() above, which
// can push onto the stack and reallocate it.
for (;;)
{
container_frame& top = container_stack.back();
if (top.remaining != npos)
{
if (top.remaining != 0)
{
--top.remaining;
if (top.is_object)
{
key.clear();
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_string(key) || !sax->key(key)))
{
return false;
}
}
// an optimized container gives its elements no marker
prefix = (top.type_marker != 0) ? top.type_marker : get_ignore_noop();
break;
}
}
else if (current != (top.is_object ? '}' : ']'))
{
// a container that ends at a marker is never optimized, so
// every element carries its own marker; for an object the
// byte tested above is the first byte of the key
if (top.is_object)
{
key.clear();
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_string(key, false) || !sax->key(key)))
{
return false;
}
prefix = get_ignore_noop();
}
else
{
prefix = current;
}
break;
}
const bool is_object = top.is_object;
container_stack.pop_back();
if (JSON_HEDLEY_UNLIKELY(is_object ? !sax->end_object() : !sax->end_array()))
{
return false;
}
if (container_stack.empty())
{
return true;
}
// the container that just ended was an element of the one
// below it, which may need the next byte for its own test
if (container_stack.back().remaining == npos)
{
get_ignore_noop();
}
}
}
}
/*!
@@ -13617,53 +13715,22 @@ class binary_reader
exception_message(input_format, "excessive array size", "size"), nullptr));
}
if (JSON_HEDLEY_UNLIKELY(!sax->start_array(size_and_type.first)))
if (JSON_HEDLEY_UNLIKELY(!enter_container(/*is_object*/false, size_and_type.first, size_and_type.second)))
{
return false;
}
if (size_and_type.second != 0)
if (size_and_type.second == 'N')
{
if (size_and_type.second != 'N')
{
for (std::size_t i = 0; i < size_and_type.first; ++i)
{
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_value(size_and_type.second)))
{
return false;
}
}
}
}
else
{
for (std::size_t i = 0; i < size_and_type.first; ++i)
{
if (JSON_HEDLEY_UNLIKELY(!parse_ubjson_internal()))
{
return false;
}
}
}
}
else
{
if (JSON_HEDLEY_UNLIKELY(!sax->start_array(detail::unknown_size())))
{
return false;
// a no-op is not a value, so a container of them holds none;
// the declared size has already been passed to the SAX parser
container_stack.back().remaining = 0;
}
while (current != ']')
{
if (JSON_HEDLEY_UNLIKELY(!parse_ubjson_internal(false)))
{
return false;
}
get_ignore_noop();
}
return true;
}
return sax->end_array();
return enter_container(/*is_object*/false, detail::unknown_size());
}
/*!
@@ -13685,68 +13752,12 @@ class binary_reader
exception_message(input_format, "BJData object does not support ND-array size in optimized format", "object"), nullptr));
}
string_t key;
if (size_and_type.first != npos)
{
if (JSON_HEDLEY_UNLIKELY(!sax->start_object(size_and_type.first)))
{
return false;
}
if (size_and_type.second != 0)
{
for (std::size_t i = 0; i < size_and_type.first; ++i)
{
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_string(key) || !sax->key(key)))
{
return false;
}
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_value(size_and_type.second)))
{
return false;
}
key.clear();
}
}
else
{
for (std::size_t i = 0; i < size_and_type.first; ++i)
{
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_string(key) || !sax->key(key)))
{
return false;
}
if (JSON_HEDLEY_UNLIKELY(!parse_ubjson_internal()))
{
return false;
}
key.clear();
}
}
}
else
{
if (JSON_HEDLEY_UNLIKELY(!sax->start_object(detail::unknown_size())))
{
return false;
}
while (current != '}')
{
if (JSON_HEDLEY_UNLIKELY(!get_ubjson_string(key, false) || !sax->key(key)))
{
return false;
}
if (JSON_HEDLEY_UNLIKELY(!parse_ubjson_internal()))
{
return false;
}
get_ignore_noop();
key.clear();
}
return enter_container(/*is_object*/true, size_and_type.first, size_and_type.second);
}
return sax->end_object();
return enter_container(/*is_object*/true, detail::unknown_size());
}
// Note, no reader for UBJSON binary types is implemented because they do