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litebox_shim_linux: fix race in prepare_for_exit child TID write #1155

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litebox_shim_linux: fix race in prepare_for_exit child TID write #1155

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10 changes: 8 additions & 2 deletions litebox_shim_linux/src/syscalls/process.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -499,8 +499,6 @@ fn wake_robust_list(
impl<FS: ShimFS> Task<FS> {
/// Called when the task is exiting.
pub(crate) fn prepare_for_exit(&mut self) {
self.thread.detach_from_process();

if let Some(clear_child_tid) = self.thread.clear_child_tid.take() {
// Clear the child TID if requested
// TODO: if we are the last thread, we don't need to clear it
Expand All @@ -516,6 +514,14 @@ impl<FS: ShimFS> Task<FS> {
if let Some(robust_list) = self.thread.robust_list.take() {
let _ = wake_robust_list(robust_list);
}

// This must run last, after all of this task's remaining accesses to
// guest memory have completed. Otherwise, because this decrements the
// `nr_threads` counter, other tasks may assume that the memory of this
// task isn't referenced any more, and therefore can be de-allocated and
// re-used (e.g., for an `exec`d process). See
// https://github.com/microsoft/litebox/pull/1155.
self.thread.detach_from_process();
}

pub(crate) fn sys_exit(&self, status: i32) {
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