[lldb] Implement Process::ReadMemoryRanges

lldb/include/lldb/Target/Process.h

@@ -1571,6 +1571,28 @@ class Process : public std::enable_shared_from_this<Process>,
   virtual size_t ReadMemory(lldb::addr_t vm_addr, void *buf, size_t size,
                             Status &error);
 
+  /// Read from multiple memory ranges and write the results into buffer.
+  /// This calls ReadMemoryFromInferior multiple times, once per range,
+  /// bypassing the read cache. Process implementations that can perform this
+  /// operation more efficiently should override this.
+  ///
+  /// \param[in] ranges
+  ///     A collection of ranges (base address + size) to read from.
+  ///
+  /// \param[out] buffer
+  ///     A buffer where the read memory will be written to. It must be at least
+  ///     as long as the sum of the sizes of each range.
+  ///
+  /// \return
+  ///     A vector of MutableArrayRef, where each MutableArrayRef is a slice of
+  ///     the input buffer into which the memory contents were copied. The size
+  ///     of the slice indicates how many bytes were read successfully. Partial
+  ///     reads are always performed from the start of the requested range,
+  ///     never from the middle or end.
+  virtual llvm::SmallVector<llvm::MutableArrayRef<uint8_t>>
+  ReadMemoryRanges(llvm::ArrayRef<Range<lldb::addr_t, size_t>> ranges,
+                   llvm::MutableArrayRef<uint8_t> buffer);
+
   /// Read of memory from a process.
   ///
   /// This function has the same semantics of ReadMemory except that it

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp

@@ -279,22 +279,23 @@ ClassDescriptorV2::ReadMethods(llvm::ArrayRef<lldb::addr_t> addresses,
   const size_t num_methods = addresses.size();
 
   llvm::SmallVector<uint8_t, 0> buffer(num_methods * size, 0);
-  llvm::DenseSet<uint32_t> failed_indices;
 
-  for (auto [idx, addr] : llvm::enumerate(addresses)) {
-    Status error;
-    process->ReadMemory(addr, buffer.data() + idx * size, size, error);
-    if (error.Fail())
-      failed_indices.insert(idx);
-  }
+  llvm::SmallVector<Range<addr_t, size_t>> mem_ranges =
+      llvm::to_vector(llvm::map_range(llvm::seq(num_methods), [&](size_t idx) {
+        return Range<addr_t, size_t>(addresses[idx], size);
+      }));
+
+  llvm::SmallVector<llvm::MutableArrayRef<uint8_t>> read_results =
+      process->ReadMemoryRanges(mem_ranges, buffer);
 
   llvm::SmallVector<method_t, 0> methods;
   methods.reserve(num_methods);
-  for (auto [idx, addr] : llvm::enumerate(addresses)) {
-    if (failed_indices.contains(idx))
+  for (auto [addr, memory] : llvm::zip(addresses, read_results)) {
+    // Ignore partial reads.
+    if (memory.size() != size)
       continue;
-    DataExtractor extractor(buffer.data() + idx * size, size,
-                            process->GetByteOrder(),
+
+    DataExtractor extractor(memory.data(), size, process->GetByteOrder(),
                             process->GetAddressByteSize());
     methods.push_back(method_t());
     methods.back().Read(extractor, process, addr, relative_selector_base_addr,

lldb/source/Target/Process.cpp

@@ -1971,6 +1971,45 @@ size_t Process::ReadMemory(addr_t addr, void *buf, size_t size, Status &error) {
   }
 }
 
+llvm::SmallVector<llvm::MutableArrayRef<uint8_t>>
+Process::ReadMemoryRanges(llvm::ArrayRef<Range<lldb::addr_t, size_t>> ranges,
+                          llvm::MutableArrayRef<uint8_t> buffer) {
+  auto total_ranges_len = llvm::sum_of(
+      llvm::map_range(ranges, [](auto range) { return range.size; }));
+  // If the buffer is not large enough, this is a programmer error.
+  // In production builds, gracefully fail by returning empty chunks.
+  assert(buffer.size() >= total_ranges_len);
+  if (buffer.size() < total_ranges_len)
+    return llvm::SmallVector<llvm::MutableArrayRef<uint8_t>>(ranges.size());
+
+  llvm::SmallVector<llvm::MutableArrayRef<uint8_t>> results;
+
+  // While `buffer` has space, take the next requested range and read
+  // memory into a `buffer` chunk, then slice it to remove the used chunk.
+  for (auto [addr, range_len] : ranges) {
+    Status status;
+    size_t num_bytes_read =
+        ReadMemoryFromInferior(addr, buffer.data(), range_len, status);
+    // FIXME: ReadMemoryFromInferior promises to return 0 in case of errors, but
+    // it doesn't; it never checks for errors.
+    if (status.Fail())
+      num_bytes_read = 0;
+
+    assert(num_bytes_read <= range_len && "read more than requested bytes");
+    if (num_bytes_read > range_len) {
+      // In production builds, gracefully fail by returning an empty chunk.
+      results.emplace_back();
+      continue;
+    }
+
+    results.push_back(buffer.take_front(num_bytes_read));
+    // Slice buffer to remove the used chunk.
+    buffer = buffer.drop_front(num_bytes_read);
+  }
+
+  return results;
+}
+
 void Process::DoFindInMemory(lldb::addr_t start_addr, lldb::addr_t end_addr,
                              const uint8_t *buf, size_t size,
                              AddressRanges &matches, size_t alignment,

lldb/unittests/Target/MemoryTest.cpp

@@ -17,6 +17,7 @@
 #include "lldb/Utility/ArchSpec.h"
 #include "lldb/Utility/DataBufferHeap.h"
 #include "gtest/gtest.h"
+#include <cstdint>
 
 using namespace lldb_private;
 using namespace lldb;
@@ -225,3 +226,62 @@ TEST_F(MemoryTest, TesetMemoryCacheRead) {
                                                        // instead of using an
                                                        // old cache
 }
+
+/// A process class that, when asked to read memory from some address X, returns
+/// the least significant byte of X.
+class DummyReaderProcess : public Process {
+public:
+  size_t DoReadMemory(lldb::addr_t vm_addr, void *buf, size_t size,
+                      Status &error) override {
+    uint8_t *buffer = static_cast<uint8_t *>(buf);
+    for (size_t addr = vm_addr; addr < vm_addr + size; addr++)
+      buffer[addr - vm_addr] = static_cast<uint8_t>(addr); // LSB of addr.
+    return size;
+  }
+  // Boilerplate, nothing interesting below.
+  DummyReaderProcess(lldb::TargetSP target_sp, lldb::ListenerSP listener_sp)
+      : Process(target_sp, listener_sp) {}
+  bool CanDebug(lldb::TargetSP, bool) override { return true; }
+  Status DoDestroy() override { return {}; }
+  void RefreshStateAfterStop() override {}
+  bool DoUpdateThreadList(ThreadList &, ThreadList &) override { return false; }
+  llvm::StringRef GetPluginName() override { return "Dummy"; }
+};
+
+TEST_F(MemoryTest, TestReadMemoryRanges) {
+  ArchSpec arch("x86_64-apple-macosx-");
+
+  Platform::SetHostPlatform(PlatformRemoteMacOSX::CreateInstance(true, &arch));
+
+  DebuggerSP debugger_sp = Debugger::CreateInstance();
+  ASSERT_TRUE(debugger_sp);
+
+  TargetSP target_sp = CreateTarget(debugger_sp, arch);
+  ASSERT_TRUE(target_sp);
+
+  ListenerSP listener_sp(Listener::MakeListener("dummy"));
+  ProcessSP process_sp =
+      std::make_shared<DummyReaderProcess>(target_sp, listener_sp);
+  ASSERT_TRUE(process_sp);
+
+  DummyProcess *process = static_cast<DummyProcess *>(process_sp.get());
+  process->SetMaxReadSize(1024);
+
+  llvm::SmallVector<uint8_t, 0> buffer(1024, 0);
+
+  // Read 8 ranges of 128 bytes with arbitrary base addresses.
+  llvm::SmallVector<Range<addr_t, size_t>> ranges = {
+      {0x12345, 128},      {0x11112222, 128}, {0x77777777, 128},
+      {0xffaabbccdd, 128}, {0x0, 128},        {0x4242424242, 128},
+      {0x17171717, 128},   {0x99999, 128}};
+
+  llvm::SmallVector<llvm::MutableArrayRef<uint8_t>> read_results =
+      process->ReadMemoryRanges(ranges, buffer);
+
+  for (auto [range, memory] : llvm::zip(ranges, read_results)) {
+    ASSERT_EQ(memory.size(), 128u);
+    addr_t range_base = range.GetRangeBase();
+    for (auto [idx, byte] : llvm::enumerate(memory))
+      ASSERT_EQ(byte, static_cast<uint8_t>(range_base + idx));
+  }
+}