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Stop CBOR indefinite-length strings from recursing per chunk

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get_cbor_string() and get_cbor_binary() handled the indefinite-length forms
(0x7F and 0x5F) by calling themselves once per chunk. Each chunk therefore
cost a native stack frame, and since a chunk may itself be an indefinite-
length string, an input of repeated 0x7F bytes reached one frame per input
byte: 200,000 of them crash the process with SIGSEGV before a single byte is
rejected. This is the same defect as #5104, in a path the container-level
work does not touch.

Count the open levels instead of recursing through them. That is enough here
because every chunk is appended to the same result -- get_bytes() writes at
result.size() -- so there is no per-level state to keep. The temporary chunk
string and its copy into the result go away with the recursion.

The definite-length cases move to get_cbor_string_chunk() and
get_cbor_binary_chunk() unchanged, including their error messages, which
still name 0x7F and 0x5F because those are handled one level up.

Behaviour is unchanged. Comparing against develop over the interesting byte
sequences -- empty, single-chunk, nested, over-closed and truncated forms,
both strings and byte arrays, and an indefinite-length map key -- produces
identical values, error codes, messages and byte offsets. The 200,000-level
input now reports parse_error.110 at byte 200001 instead of crashing.

Note that nesting these is not valid CBOR: RFC 8949, Section 3.2.3 forbids
it. This does not change that either way -- it has always been accepted, and
rejecting it is a separate decision (#5317, #5325). Should it be rejected
later, that is now one condition on the level counter rather than a change to
the control flow.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-07 07:50:27 +02:00
parent 755c547003
commit 00e5d21041
3 changed files with 326 additions and 102 deletions
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tests/src/unit-cbor.cpp
+52
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@@ -2035,6 +2035,58 @@ TEST_CASE("CBOR definite length equal to the indefinite-length sentinel")
}
}
TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
{
// Reading an indefinite-length string or byte array used to call itself
// once per chunk, so a payload of repeated 0x7F (or 0x5F) bytes exhausted
// the call stack before any of the input was rejected. The open levels are
// counted now, and the levels below prove the reader still reads the same
// values and reports the same errors at the same byte offsets.
json _;
SECTION("many open levels are reported, not crashed on")
{
const std::vector<uint8_t> input(200000, 0x7F);
CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR string: unexpected end of input", json::parse_error&);
CHECK(json::from_cbor(input, true, false).is_discarded());
}
SECTION("many open levels are reported, not crashed on (binary)")
{
const std::vector<uint8_t> input(200000, 0x5F);
CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
CHECK(json::from_cbor(input, true, false).is_discarded());
}
SECTION("chunks are still concatenated")
{
CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0xFF})) == json(""));
CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 0x61, 0xFF})) == json("a"));
// nested indefinite-length strings are concatenated across levels
CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0x61, 0x62, 0xFF})) == json("ab"));
CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x7F, 0x61, 0x7A, 0xFF, 0xFF, 0xFF})) == json("z"));
CHECK(json::from_cbor(std::vector<uint8_t>({0xA1, 0x7F, 0x61, 0x61, 0xFF, 0x01})) == json({{"a", 1}}));
}
SECTION("chunks are still concatenated (binary)")
{
CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0xFF})) == json::binary({0x61}));
CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0x41, 0x62, 0xFF})) == json::binary({0x61, 0x62}));
}
SECTION("a chunk that is not a string is still rejected")
{
CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x00", json::parse_error&);
CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
}
SECTION("a break marker outside an indefinite-length string is not a string")
{
// 0xFF only closes a string that was opened; on its own it is not one
CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0xFF, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xFF", json::parse_error&);
}
}
TEST_CASE("CBOR roundtrips" * doctest::skip())
{
SECTION("input from flynn")