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Add instruction length limits.

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This commit is contained in:
Thomas Harte 2022-03-10 20:47:56 -05:00
parent 641e0c1afc
commit f92ffddb82
2 changed files with 34 additions and 13 deletions
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26
InstructionSets/x86/Decoder.cpp
View File

@ -14,15 +14,17 @@
using namespace InstructionSet::x86;
// TODO: instruction length limits:
//
// 8086/80186: none
// 80286: 10 bytes
// 80386: 15 bytes
template <Model model>
std::pair<int, typename Decoder<model>::InstructionT> Decoder<model>::decode(const uint8_t *source, size_t length) {
const uint8_t *const end = source + length;
// Instruction length limits:
//
// 8086/80186: none
// 80286: 10 bytes
// 80386: 15 bytes
constexpr int max_instruction_length = model >= Model::i80386 ? 15 : (model == Model::i80286 ? 10 : 0);
const uint8_t *const buffer_end = source + length;
const uint8_t *const end = max_instruction_length ? std::min(buffer_end, source + max_instruction_length - consumed_) : buffer_end;
// MARK: - Prefixes (if present) and the opcode.
@ -436,6 +438,7 @@ std::pair<int, typename Decoder<model>::InstructionT> Decoder<model>::decode(con
}
// MARK: - Additional F page of instructions.
if(phase_ == Phase::InstructionPageF && source != end) {
// Update the instruction acquired.
const uint8_t instr = *source;
@ -861,7 +864,7 @@ std::pair<int, typename Decoder<model>::InstructionT> Decoder<model>::decode(con
// MARK: - Displacement and operand.
if(phase_ == Phase::DisplacementOrOperand && source != end) {
if(phase_ == Phase::DisplacementOrOperand) {
const auto required_bytes = int(byte_size(displacement_size_) + byte_size(operand_size_));
const int outstanding_bytes = required_bytes - operand_bytes_;
@ -928,6 +931,13 @@ std::pair<int, typename Decoder<model>::InstructionT> Decoder<model>::decode(con
return result;
}
// Check for a too-long instruction.
if(max_instruction_length && consumed_ == max_instruction_length) {
const auto result = std::make_pair(consumed_, InstructionT());
reset_parsing();
return result;
}
// i.e. not done yet.
return std::make_pair(0, InstructionT());
}

21
InstructionSets/x86/Decoder.hpp
View File

@ -28,11 +28,22 @@ template <Model model> class Decoder {
using InstructionT = Instruction<model >= Model::i80386>;
/*!
@returns an @c Instruction plus a size; a positive size to indicate successful decoding; a
negative size specifies the [negatived] number of further bytes the caller should ideally
collect before calling again. The caller is free to call with fewer, but may not get a decoded
instruction in response, and the decoder may still not be able to complete decoding
even if given that number of bytes.
@returns an @c Instruction plus a size; a positive size indicates successful decoding of
an instruction that was that many bytes long in total; a negative size specifies the [negatived]
minimum number of further bytes the caller should ideally collect before calling again. The
caller is free to call with fewer, but may not get a decoded instruction in response, and the
decoder may still not be able to complete decoding even if given that number of bytes.
Successful decoding is defined to mean that all decoding steps are complete. The output
may still be an illegal instruction (indicated by Operation::Invalid), if the byte sequence
supplied cannot form a valid instruction.
@discussion although instructions also contain an indicator of their length, on chips prior
to the 80286 there is no limit to instruction length and that could in theory overflow the available
storage, which can describe instructions only up to 1kb in size.
The 80286 and 80386 have instruction length limits of 10 and 15 bytes respectively, so
cannot overflow the field.
*/
std::pair<int, InstructionT> decode(const uint8_t *source, size_t length);