Fixed pread-related bug

pread may or may not read the requested size. This was not
taken into account for. If the data is large,
we probably can't read it, in one go.

Author: theIDinside

src/PersistentCheckpointing.cc

@@ -67,13 +67,26 @@ WriteVmConfig::WriteVmConfig(Task* clone_leader, const char* data_dir,
   ASSERT(clone_leader, proc_pagemap_fd.is_open())
       << "Serializing VM for " << clone_leader->rec_tid
       << " failed. Couldn't open " << proc_pagemap_fd;
-  buffer = { .ptr = ::mmap(nullptr, buffer_size, PROT_READ | PROT_WRITE,
-                           MAP_PRIVATE | MAP_ANONYMOUS, -1, 0),
+  buffer = { .ptr =
+                 (uint8_t*)::mmap(nullptr, buffer_size, PROT_READ | PROT_WRITE,
+                                  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0),
              .size = buffer_size };
   ASSERT(clone_leader, buffer.ptr != MAP_FAILED)
       << "Failed to mmap buffer with capacity " << buffer_size;
 }
 
+ssize_t WriteVmConfig::pread(ssize_t bytes_read,
+                             const KernelMapping& km) const {
+  DEBUG_ASSERT(bytes_read != 1 &&
+               "you've passed in the 'invalid pread result' as bytes_read");
+  const auto current_read =
+      ::pread(proc_mem_fd, buffer.ptr + bytes_read, km.size() - bytes_read,
+              km.start().as_int() + bytes_read);
+  if (current_read == -1)
+    return current_read;
+  return bytes_read + current_read;
+}
+
 std::string checkpoints_index_file(const std::string& trace_dir) {
   return trace_dir + "/checkpoints";
 }
@@ -151,10 +164,14 @@ static void write_map(const WriteVmConfig& cfg,
       FILE_OP_FATAL(file) << "couldn't truncate file to size "
                           << map.map.size();
 
-    const auto bytes_read = ::pread(cfg.proc_mem_fd, cfg.buffer.ptr,
-                                    map.map.size(), map.map.start().as_int());
-    if (bytes_read == -1)
-      FILE_OP_FATAL(file) << " couldn't read contents of " << map.map.str();
+    auto bytes_read = 0;
+    while (static_cast<size_t>(bytes_read) < map.map.size()) {
+      const auto current_read = cfg.pread(bytes_read, map.map);
+      if (current_read == -1)
+        FILE_OP_FATAL(file) << " couldn't read contents of " << map.map.str();
+      bytes_read = current_read;
+    }
+
     ASSERT(cfg.clone_leader,
            static_cast<unsigned long>(bytes_read) == map.map.size())
         << " data read from /proc/" << cfg.clone_leader->tid

src/PersistentCheckpointing.h

@@ -28,9 +28,11 @@ class WriteVmConfig {
   const char* cp_data_dir;
 
   struct {
-    void* ptr;
+    uint8_t* ptr;
     size_t size;
   } buffer;
+
+  ssize_t pread(ssize_t bytes_read, const KernelMapping& km) const;
 };
 
 /* Writes capture `state` to state builder `sb`. */