Add a persistent pagemap_fd per Task.

Changes in V2:
- Merge open_pagemap_fd and open_pagemap_fd_if_needed
  into Task::pagemap_fd, returning the fd.

Author: bernhardu

src/AddressSpace.h

@@ -674,6 +674,9 @@ class AddressSpace : public HasTaskSet {
   ScopedFd& mem_fd() { return child_mem_fd; }
   void set_mem_fd(ScopedFd&& fd) { child_mem_fd = std::move(fd); }
 
+  ScopedFd& pagemap_fd() { return child_pagemap_fd; }
+  void set_pagemap_fd(ScopedFd&& fd) { child_pagemap_fd = std::move(fd); }
+
   Monkeypatcher& monkeypatcher() {
     DEBUG_ASSERT(monkeypatch_state);
     return *monkeypatch_state;
@@ -1209,6 +1212,8 @@ class AddressSpace : public HasTaskSet {
                                      const struct map_iterator_data* data);
 
   AddressSpace operator=(const AddressSpace&) = delete;
+
+  ScopedFd child_pagemap_fd;
 };
 
 /**

src/Task.cc

@@ -2729,6 +2729,18 @@ void Task::open_mem_fd_if_needed() {
   }
 }
 
+ScopedFd& Task::pagemap_fd() {
+  if (!as->pagemap_fd().is_open()) {
+    ScopedFd fd(proc_pagemap_path().c_str(), O_RDONLY);
+    if (fd.is_open()) {
+      as->set_pagemap_fd(std::move(fd));
+    } else {
+      LOG(info) << "Can't retrieve pagemap fd for " << tid;
+    }
+  }
+  return as->pagemap_fd();
+}
+
 KernelMapping Task::init_syscall_buffer(AutoRemoteSyscalls& remote,
                                         remote_ptr<void> map_hint) {
   char name[50];

src/Task.h

@@ -820,6 +820,11 @@ class Task {
    */
   void open_mem_fd_if_needed();
 
+  /**
+   * Open /proc/[tid]/pagemap fd for our AddressSpace.
+   */
+  ScopedFd& pagemap_fd();
+
   /**
    * Perform a PTRACE_INTERRUPT set up the counter for potential spurious stops
    * to be detected in `account_for_potential_ptrace_interrupt_stop`.

src/record_syscall.cc

@@ -5656,7 +5656,7 @@ static void process_execve(RecordTask* t, TaskSyscallState& syscall_state) {
   // The kernel may modify some of the pages in the mapping according to
   // ELF BSS metadata. We use /proc/<pid>/pagemap to observe which pages
   // have been changed and mark them for recording.
-  ScopedFd pagemap(t->proc_pagemap_path().c_str(), O_RDONLY, 0);
+  ScopedFd& pagemap = t->pagemap_fd();
   ASSERT(t, pagemap.is_open());
   vector<remote_ptr<void>> pages_to_record;