Add explanation for clone use
Test: no code changes
Change-Id: I70c8af8261cda3163677557fd78ae0119b1f1be0
diff --git a/libmemunreachable/PtracerThread.cpp b/libmemunreachable/PtracerThread.cpp
index aca2a82..61a1d24 100644
--- a/libmemunreachable/PtracerThread.cpp
+++ b/libmemunreachable/PtracerThread.cpp
@@ -98,6 +98,7 @@
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) {
diff --git a/libmemunreachable/README.md b/libmemunreachable/README.md
index 61a47de..ae8fa94 100644
--- a/libmemunreachable/README.md
+++ b/libmemunreachable/README.md
@@ -36,7 +36,7 @@
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()`.