Merge "Add explanation for clone use"

libmemunreachable/PtracerThread.cpp

@@ -98,6 +98,7 @@ bool PtracerThread::Start() {
     return (*reinterpret_cast<std::function<int()>*>(arg))();
   };
 
+  // See README.md for why we create the child process this way
   child_pid_ = clone(proxy, stack_->top(), CLONE_VM | CLONE_FS | CLONE_FILES /*|CLONE_UNTRACED*/,
                      reinterpret_cast<void*>(&func_));
   if (child_pid_ < 0) {

libmemunreachable/README.md

@@ -36,7 +36,7 @@ The sequence of steps required to perform a leak detection pass is divided into
 
  1. *Original process*: Leak detection is requested by calling `GetUnreachableMemory()`
  2. Allocations are disabled using `malloc_disable()`
- 3. The collection process is spawned.  The collection process is similar to a normal `fork()` child process, except that it shares the address space of the parent - any writes by the original process are visible to the collection process, and vice-versa.
+ 3. The collection process is spawned.  The collection process, created using clone, is similar to a normal `fork()` child process, except that it shares the address space of the parent - any writes by the original process are visible to the collection process, and vice-versa. If we forked instead of using clone, the address space might get out of sync with observed post-ptrace thread state, since it takes some time to pause the parent.
  4. *Collection process*: All threads in the original process are paused with `ptrace()`.
  5. Registers contents, active stack areas, and memory mapping information are collected.
  6. *Original process*: Allocations are re-enabled using `malloc_enable()`, but all threads are still paused with `ptrace()`.