Change Target::ReadMemory to ensure the amount of memory read from the file-cache is the amount requested.

This change ensures that if for whatever reason we read less bytes than expected (for example, when trying to read memory that spans multiple sections), we try reading from the live process as well.

Reviewed By: jasonmolenda

Differential Revision: https://reviews.llvm.org/D101390

(cherry picked from commit 6c82b8a)

Author: augusto2112

lldb/source/Target/Target.cpp

@@ -1758,19 +1758,30 @@ size_t Target::ReadMemory(const Address &addr, void *dst, size_t dst_len,
   if (!resolved_addr.IsValid())
     resolved_addr = addr;
 
-  bool is_readonly = false;
+  // If we read from the file cache but can't get as many bytes as requested,
+  // we keep the result around in this buffer, in case this result is the
+  // best we can do.
+  std::unique_ptr<uint8_t[]> file_cache_read_buffer;
+  size_t file_cache_bytes_read = 0;
+
   // Read from file cache if read-only section.
   if (!force_live_memory && resolved_addr.IsSectionOffset()) {
     SectionSP section_sp(resolved_addr.GetSection());
     if (section_sp) {
       auto permissions = Flags(section_sp->GetPermissions());
-      is_readonly = !permissions.Test(ePermissionsWritable) &&
-                    permissions.Test(ePermissionsReadable);
-    }
-    if (is_readonly) {
-      bytes_read = ReadMemoryFromFileCache(resolved_addr, dst, dst_len, error);
-      if (bytes_read > 0)
-        return bytes_read;
+      bool is_readonly = !permissions.Test(ePermissionsWritable) &&
+                         permissions.Test(ePermissionsReadable);
+      if (is_readonly) {
+        file_cache_bytes_read =
+            ReadMemoryFromFileCache(resolved_addr, dst, dst_len, error);
+        if (file_cache_bytes_read == dst_len)
+          return file_cache_bytes_read;
+        else if (file_cache_bytes_read > 0) {
+          file_cache_read_buffer =
+              std::make_unique<uint8_t[]>(file_cache_bytes_read);
+          std::memcpy(file_cache_read_buffer.get(), dst, file_cache_bytes_read);
+        }
+      }
     }
   }
 
@@ -1809,7 +1820,14 @@ size_t Target::ReadMemory(const Address &addr, void *dst, size_t dst_len,
     }
   }
 
-  if (!is_readonly && resolved_addr.IsSectionOffset()) {
+  if (file_cache_read_buffer && file_cache_bytes_read > 0) {
+    // Reading from the process failed. If we've previously succeeded in reading
+    // something from the file cache, then copy that over and return that.
+    std::memcpy(dst, file_cache_read_buffer.get(), file_cache_bytes_read);
+    return file_cache_bytes_read;
+  }
+
+  if (!file_cache_read_buffer && resolved_addr.IsSectionOffset()) {
     // If we didn't already try and read from the object file cache, then try
     // it after failing to read from the process.
     return ReadMemoryFromFileCache(resolved_addr, dst, dst_len, error);