[Lldb-commits] [lldb] [lldb] Don't build an opcode from a failed disassembly (PR #208279)
Charles Zablit via lldb-commits
lldb-commits at lists.llvm.org
Thu Jul 9 09:49:46 PDT 2026
https://github.com/charles-zablit updated https://github.com/llvm/llvm-project/pull/208279
>From efa86ebdbba4960f663969c4004317a5001d64ec Mon Sep 17 00:00:00 2001
From: Charles Zablit <c_zablit at apple.com>
Date: Wed, 8 Jul 2026 18:53:21 +0100
Subject: [PATCH 1/2] [lldb] Don't build an opcode from a failed disassembly
---
lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp b/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp
index a18c95bdbde98..fe59e988e3cd9 100644
--- a/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp
+++ b/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp
@@ -530,7 +530,7 @@ class InstructionLLVMC : public lldb_private::Instruction {
m_is_valid = mc_disasm_ptr->GetMCInst(opcode_data, opcode_data_len,
pc, inst, inst_size);
m_opcode.Clear();
- if (inst_size != 0) {
+ if (m_is_valid && inst_size != 0) {
if (arch.GetTriple().isRISCV())
m_opcode.SetOpcode16_32TupleBytes(opcode_data, inst_size,
byte_order);
>From 946b826afd082c2a36b4341f4bef82c56359c9ad Mon Sep 17 00:00:00 2001
From: Charles Zablit <c_zablit at apple.com>
Date: Thu, 9 Jul 2026 17:49:34 +0100
Subject: [PATCH 2/2] fixup! [lldb] Don't build an opcode from a failed
disassembly
---
lldb/include/lldb/Core/Opcode.h | 9 +-
.../Disassembler/LLVMC/DisassemblerLLVMC.cpp | 5 +-
lldb/unittests/Disassembler/CMakeLists.txt | 1 +
.../x86/TestGetOpcodeOversized.cpp | 90 +++++++++++++++++++
4 files changed, 102 insertions(+), 3 deletions(-)
create mode 100644 lldb/unittests/Disassembler/x86/TestGetOpcodeOversized.cpp
diff --git a/lldb/include/lldb/Core/Opcode.h b/lldb/include/lldb/Core/Opcode.h
index 7e756d3f15d22..2738e71a207da 100644
--- a/lldb/include/lldb/Core/Opcode.h
+++ b/lldb/include/lldb/Core/Opcode.h
@@ -43,6 +43,11 @@ class Opcode {
Opcode() = default;
+ /// The size in bytes of the fixed buffer used to store opcode bytes. It must
+ /// be big enough to hold the longest opcode of any supported target (x86
+ /// caps instructions at 15 bytes).
+ static constexpr size_t kMaxByteSize = 16;
+
Opcode(uint8_t inst, lldb::ByteOrder order)
: m_byte_order(order), m_type(eType8) {
m_data.inst8 = inst;
@@ -294,8 +299,8 @@ class Opcode {
uint32_t inst32;
uint64_t inst64;
struct {
- uint8_t bytes[16]; // This must be big enough to handle any opcode for any
- // supported target.
+ uint8_t bytes[kMaxByteSize]; // This must be big enough to handle any
+ // opcode for any supported target.
uint8_t length;
} inst;
} m_data;
diff --git a/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp b/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp
index fe59e988e3cd9..6129967ce65bc 100644
--- a/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp
+++ b/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp
@@ -45,6 +45,8 @@
#include "lldb/Utility/Log.h"
#include "lldb/Utility/RegularExpression.h"
#include "lldb/Utility/Stream.h"
+
+#include <algorithm>
#include <optional>
using namespace lldb;
@@ -530,7 +532,8 @@ class InstructionLLVMC : public lldb_private::Instruction {
m_is_valid = mc_disasm_ptr->GetMCInst(opcode_data, opcode_data_len,
pc, inst, inst_size);
m_opcode.Clear();
- if (m_is_valid && inst_size != 0) {
+ inst_size = std::min<uint64_t>(inst_size, Opcode::kMaxByteSize);
+ if (inst_size != 0) {
if (arch.GetTriple().isRISCV())
m_opcode.SetOpcode16_32TupleBytes(opcode_data, inst_size,
byte_order);
diff --git a/lldb/unittests/Disassembler/CMakeLists.txt b/lldb/unittests/Disassembler/CMakeLists.txt
index 0f6d21178933c..8894bcfa8f081 100644
--- a/lldb/unittests/Disassembler/CMakeLists.txt
+++ b/lldb/unittests/Disassembler/CMakeLists.txt
@@ -10,6 +10,7 @@ endif()
if("X86" IN_LIST LLVM_TARGETS_TO_BUILD)
list(APPEND disas_srcs
x86/TestGetControlFlowKindx86.cpp
+ x86/TestGetOpcodeOversized.cpp
)
endif()
diff --git a/lldb/unittests/Disassembler/x86/TestGetOpcodeOversized.cpp b/lldb/unittests/Disassembler/x86/TestGetOpcodeOversized.cpp
new file mode 100644
index 0000000000000..793d221634958
--- /dev/null
+++ b/lldb/unittests/Disassembler/x86/TestGetOpcodeOversized.cpp
@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/TargetSelect.h"
+#include "gtest/gtest.h"
+
+#include "lldb/Core/Address.h"
+#include "lldb/Core/Disassembler.h"
+#include "lldb/Core/Opcode.h"
+#include "lldb/Utility/ArchSpec.h"
+
+#include "Plugins/Disassembler/LLVMC/DisassemblerLLVMC.h"
+
+using namespace lldb;
+using namespace lldb_private;
+
+namespace {
+class TestGetOpcodeOversized : public testing::Test {
+public:
+ static void SetUpTestCase();
+ static void TearDownTestCase();
+};
+
+void TestGetOpcodeOversized::SetUpTestCase() {
+ llvm::InitializeAllTargets();
+ llvm::InitializeAllAsmPrinters();
+ llvm::InitializeAllTargetMCs();
+ llvm::InitializeAllDisassemblers();
+ DisassemblerLLVMC::Initialize();
+}
+
+void TestGetOpcodeOversized::TearDownTestCase() {
+ DisassemblerLLVMC::Terminate();
+}
+} // namespace
+
+// A long enough run of redundant prefixes makes the x86 disassembler report an
+// instruction whose length is greater than Opcode's fixed-size byte buffer.
+TEST_F(TestGetOpcodeOversized, OversizedInstructionIsTruncated) {
+ ArchSpec arch("x86_64-*-linux");
+
+ // 20 operand-size (0x66) prefixes followed by a NOP: decodes successfully as
+ // a 21-byte instruction, overlowing the 15-byte x86 limit and the 16-byte
+ // opcode buffer.
+ uint8_t data[21];
+ memset(data, 0x66, sizeof(data));
+ data[sizeof(data) - 1] = 0x90; // nop
+ static_assert(sizeof(data) > Opcode::kMaxByteSize,
+ "test input must exceed the opcode buffer");
+
+ Address start_addr(0x100);
+ DisassemblerSP disass_sp = Disassembler::DisassembleBytes(
+ arch, nullptr, nullptr, nullptr, nullptr, start_addr, &data, sizeof(data),
+ /*num_instructions=*/1, false);
+
+ if (!disass_sp)
+ return;
+
+ const InstructionList &inst_list = disass_sp->GetInstructionList();
+ ASSERT_EQ(1u, inst_list.GetSize());
+
+ // The instruction is still produced, but its recorded opcode is clamped to
+ // the buffer capacity rather than overflowing it.
+ InstructionSP inst_sp = inst_list.GetInstructionAtIndex(0);
+ EXPECT_LE(inst_sp->GetOpcode().GetByteSize(), Opcode::kMaxByteSize);
+}
+
+// A normal instruction is unaffected by the truncation logic.
+TEST_F(TestGetOpcodeOversized, NormalInstructionIsUnchanged) {
+ ArchSpec arch("x86_64-*-linux");
+
+ uint8_t data[] = {0x48, 0x83, 0xec, 0x18}; // subq $0x18, %rsp
+
+ Address start_addr(0x100);
+ DisassemblerSP disass_sp = Disassembler::DisassembleBytes(
+ arch, nullptr, nullptr, nullptr, nullptr, start_addr, &data, sizeof(data),
+ /*num_instructions=*/1, false);
+
+ if (!disass_sp)
+ return;
+
+ const InstructionList &inst_list = disass_sp->GetInstructionList();
+ ASSERT_EQ(1u, inst_list.GetSize());
+ EXPECT_EQ(4u, inst_list.GetInstructionAtIndex(0)->GetOpcode().GetByteSize());
+}
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