[LLDB] Fix AddressSanitizer failure in MemoryCache

The lldb sanitizer bot is flagging a container-overflow error after we
introduced test TestWasm.py. MemoryCache::Read didn't behave correctly
in case of partial reads that can happen with object files whose size is
smaller that the cache size. It should return the actual number of bytes
read and not try to fill the buffer with random memory.
Module::GetMemoryObjectFile needs to be modified accordingly, to resize
its buffer to only the size that was read.

Differential Revision: https://reviews.llvm.org/D75200

Author: paolosevMSFT

lldb/source/Core/Module.cpp

@@ -297,7 +297,9 @@ ObjectFile *Module::GetMemoryObjectFile(const lldb::ProcessSP &process_sp,
       const size_t bytes_read =
           process_sp->ReadMemory(header_addr, data_up->GetBytes(),
                                  data_up->GetByteSize(), readmem_error);
-      if (bytes_read == size_to_read) {
+      if (bytes_read < size_to_read)
+        data_up->SetByteSize(bytes_read);
+      if (data_up->GetByteSize() > 0) {
         DataBufferSP data_sp(data_up.release());
         m_objfile_sp = ObjectFile::FindPlugin(shared_from_this(), process_sp,
                                               header_addr, data_sp);

lldb/source/Target/Memory.cpp

@@ -232,8 +232,13 @@ size_t MemoryCache::Read(addr_t addr, void *dst, size_t dst_len,
         if (process_bytes_read == 0)
           return dst_len - bytes_left;
 
-        if (process_bytes_read != cache_line_byte_size)
+        if (process_bytes_read != cache_line_byte_size) {
+          if (process_bytes_read < data_buffer_heap_up->GetByteSize()) {
+            dst_len -= data_buffer_heap_up->GetByteSize() - process_bytes_read;
+            bytes_left = process_bytes_read;
+          }
           data_buffer_heap_up->SetByteSize(process_bytes_read);
+        }
         m_L2_cache[curr_addr] = DataBufferSP(data_buffer_heap_up.release());
         // We have read data and put it into the cache, continue through the
         // loop again to get the data out of the cache...