Fix a bug where lldb does not respect the packet size.

Summary: LLDB was using packet size advertised by the target as the max memory size to write in one go. It is wrong because packets have other overhead apart from memory payload. Also memory transferred through 'm' and 'M' packets needs 2 bytes in packet to transfer 1 of memory.

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D28808

llvm-svn: 292987

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

@@ -2731,16 +2731,19 @@ void ProcessGDBRemote::WillPublicStop() {
 size_t ProcessGDBRemote::DoReadMemory(addr_t addr, void *buf, size_t size,
                                       Error &error) {
   GetMaxMemorySize();
-  if (size > m_max_memory_size) {
+  bool binary_memory_read = m_gdb_comm.GetxPacketSupported();
+  // M and m packets take 2 bytes for 1 byte of memory
+  size_t max_memory_size =
+      binary_memory_read ? m_max_memory_size : m_max_memory_size / 2;
+  if (size > max_memory_size) {
     // Keep memory read sizes down to a sane limit. This function will be
     // called multiple times in order to complete the task by
     // lldb_private::Process so it is ok to do this.
-    size = m_max_memory_size;
+    size = max_memory_size;
   }
 
   char packet[64];
   int packet_len;
-  bool binary_memory_read = m_gdb_comm.GetxPacketSupported();
   packet_len = ::snprintf(packet, sizeof(packet), "%c%" PRIx64 ",%" PRIx64,
                           binary_memory_read ? 'x' : 'm', (uint64_t)addr,
                           (uint64_t)size);
@@ -2787,11 +2790,13 @@ size_t ProcessGDBRemote::DoReadMemory(addr_t addr, void *buf, size_t size,
 size_t ProcessGDBRemote::DoWriteMemory(addr_t addr, const void *buf,
                                        size_t size, Error &error) {
   GetMaxMemorySize();
-  if (size > m_max_memory_size) {
+  // M and m packets take 2 bytes for 1 byte of memory
+  size_t max_memory_size = m_max_memory_size / 2;
+  if (size > max_memory_size) {
     // Keep memory read sizes down to a sane limit. This function will be
     // called multiple times in order to complete the task by
     // lldb_private::Process so it is ok to do this.
-    size = m_max_memory_size;
+    size = max_memory_size;
   }
 
   StreamString packet;
@@ -3990,6 +3995,21 @@ void ProcessGDBRemote::GetMaxMemorySize() {
         stub_max_size = reasonable_largeish_default;
       }
 
+      // Memory packet have other overheads too like Maddr,size:#NN
+      // Instead of calculating the bytes taken by size and addr every
+      // time, we take a maximum guess here.
+      if (stub_max_size > 70)
+        stub_max_size -= 32 + 32 + 6;
+      else {
+        // In unlikely scenario that max packet size is less then 70, we will
+        // hope that data being written is small enough to fit.
+        Log *log(ProcessGDBRemoteLog::GetLogIfAnyCategoryIsSet(
+            GDBR_LOG_COMM | GDBR_LOG_MEMORY));
+        if (log)
+          log->Warning("Packet size is too small. "
+                       "LLDB may face problems while writing memory");
+      }
+
       m_max_memory_size = stub_max_size;
     } else {
       m_max_memory_size = conservative_default;