[lldb] Fix nullptr dereference on running x86 binary with x86-disabled llvm (#82603)

If `LLVM_TARGETS_TO_BUILD` does not contain `X86` and we try to run an
x86 binary in lldb, we get a `nullptr` dereference in
`LLVMDisasmInstruction(...)`. We try to call `getDisAsm()` method on a
`LLVMDisasmContext *DC` which is null. The pointer is passed from
`x86AssemblyInspectionEngine::instruction_length(...)` and is originally
`m_disasm_context` member of `x86AssemblyInspectionEngine`. This should
be filled by `LLVMCreateDisasm(...)` in the class constructor, but not
having X86 target enabled in llvm makes
`TargetRegistry::lookupTarget(...)` call return `nullptr`, which results
in `m_disasm_context` initialized with `nullptr` as well.

This patch adds if statements against `m_disasm_context` in
`x86AssemblyInspectionEngine::GetNonCallSiteUnwindPlanFromAssembly(...)`
and `x86AssemblyInspectionEngine::FindFirstNonPrologueInstruction(...)`
so subsequent calls to
`x86AssemblyInspectionEngine::instruction_length(...)` do not cause a
null pointer dereference.

Author: kovdan01

lldb/source/Plugins/UnwindAssembly/x86/x86AssemblyInspectionEngine.cpp

@@ -909,6 +909,9 @@ bool x86AssemblyInspectionEngine::GetNonCallSiteUnwindPlanFromAssembly(
   if (!m_register_map_initialized)
     return false;
 
+  if (m_disasm_context == nullptr)
+    return false;
+
   addr_t current_func_text_offset = 0;
   int current_sp_bytes_offset_from_fa = 0;
   bool is_aligned = false;
@@ -1570,6 +1573,9 @@ bool x86AssemblyInspectionEngine::FindFirstNonPrologueInstruction(
   if (!m_register_map_initialized)
     return false;
 
+  if (m_disasm_context == nullptr)
+    return false;
+
   while (offset < size) {
     int regno;
     int insn_len;

lldb/unittests/UnwindAssembly/CMakeLists.txt

@@ -9,3 +9,7 @@ endif()
 if ("X86" IN_LIST LLVM_TARGETS_TO_BUILD)
   add_subdirectory(x86)
 endif()
+
+if (NOT "X86" IN_LIST LLVM_TARGETS_TO_BUILD)
+  add_subdirectory(x86-but-no-x86-target)
+endif()

lldb/unittests/UnwindAssembly/x86-but-no-x86-target/CMakeLists.txt

@@ -0,0 +1,10 @@
+add_lldb_unittest(UnwindAssemblyX86ButNoX86TargetTests
+    Testx86AssemblyInspectionEngine.cpp
+    LINK_LIBS
+      lldbCore
+      lldbSymbol
+      lldbPluginUnwindAssemblyX86
+    LINK_COMPONENTS
+      Support
+      ${LLVM_TARGETS_TO_BUILD}
+  )

lldb/unittests/UnwindAssembly/x86-but-no-x86-target/Testx86AssemblyInspectionEngine.cpp

@@ -0,0 +1,103 @@
+//===-- Testx86AssemblyInspectionEngine.cpp -------------------------------===//
+
+//
+// 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 "gtest/gtest.h"
+
+#include "Plugins/UnwindAssembly/x86/x86AssemblyInspectionEngine.h"
+#include "lldb/Core/AddressRange.h"
+#include "lldb/Symbol/UnwindPlan.h"
+#include "lldb/Utility/ArchSpec.h"
+
+#include "llvm/Support/TargetSelect.h"
+
+#include <memory>
+#include <vector>
+
+using namespace lldb;
+using namespace lldb_private;
+
+class Testx86AssemblyInspectionEngine : public testing::Test {
+public:
+  static void SetUpTestCase();
+};
+
+void Testx86AssemblyInspectionEngine::SetUpTestCase() {
+  llvm::InitializeAllTargets();
+  llvm::InitializeAllAsmPrinters();
+  llvm::InitializeAllTargetMCs();
+  llvm::InitializeAllDisassemblers();
+}
+
+// only defining the register names / numbers that the unwinder is actually
+// using today
+
+// names should match the constants below.  These will be the eRegisterKindLLDB
+// register numbers.
+
+const char *x86_64_reg_names[] = {"rax", "rbx", "rcx", "rdx", "rsp", "rbp",
+                                  "rsi", "rdi", "r8",  "r9",  "r10", "r11",
+                                  "r12", "r13", "r14", "r15", "rip"};
+
+enum x86_64_regs {
+  k_rax = 0,
+  k_rbx = 1,
+  k_rcx = 2,
+  k_rdx = 3,
+  k_rsp = 4,
+  k_rbp = 5,
+  k_rsi = 6,
+  k_rdi = 7,
+  k_r8 = 8,
+  k_r9 = 9,
+  k_r10 = 10,
+  k_r11 = 11,
+  k_r12 = 12,
+  k_r13 = 13,
+  k_r14 = 14,
+  k_r15 = 15,
+  k_rip = 16
+};
+
+std::unique_ptr<x86AssemblyInspectionEngine> Getx86_64Inspector() {
+
+  ArchSpec arch("x86_64-apple-macosx");
+  std::unique_ptr<x86AssemblyInspectionEngine> engine(
+      new x86AssemblyInspectionEngine(arch));
+
+  std::vector<x86AssemblyInspectionEngine::lldb_reg_info> lldb_regnums;
+  int i = 0;
+  for (const auto &name : x86_64_reg_names) {
+    x86AssemblyInspectionEngine::lldb_reg_info ri;
+    ri.name = name;
+    ri.lldb_regnum = i++;
+    lldb_regnums.push_back(ri);
+  }
+
+  engine->Initialize(lldb_regnums);
+  return engine;
+}
+
+TEST_F(Testx86AssemblyInspectionEngine, TestSimple64bitFrameFunction) {
+  std::unique_ptr<x86AssemblyInspectionEngine> engine = Getx86_64Inspector();
+
+  // 'int main() { }' compiled for x86_64-apple-macosx with clang
+  uint8_t data[] = {
+      0x55,             // offset 0 -- pushq %rbp
+      0x48, 0x89, 0xe5, // offset 1 -- movq %rsp, %rbp
+      0x31, 0xc0,       // offset 4 -- xorl %eax, %eax
+      0x5d,             // offset 6 -- popq %rbp
+      0xc3              // offset 7 -- retq
+  };
+
+  AddressRange sample_range(0x1000, sizeof(data));
+
+  UnwindPlan unwind_plan(eRegisterKindLLDB);
+  EXPECT_FALSE(engine->GetNonCallSiteUnwindPlanFromAssembly(
+      data, sizeof(data), sample_range, unwind_plan));
+}