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uprobes: Ensure return_instance is detached from the list before freeing
Ensure that by the time we call free_ret_instance() to clean up an instance of struct return_instance it isn't reachable from utask->return_instances anymore. free_ret_instance() is called in a few different situations, all but one of which already are fine w.r.t. return_instance visibility: - uprobe_free_utask() guarantees that ri_timer() won't be called (through timer_delete_sync() call), and so there is no need to unlink anything, because entire utask is being freed; - uprobe_handle_trampoline() is already unlinking to-be-freed return_instance with rcu_assign_pointer() before calling free_ret_instance(). Only cleanup_return_instances() violates this property, which so far is not causing problems due to RCU-delayed freeing of return_instance, which we'll change in the next patch. So make sure we unlink return_instance before passing it into free_ret_instance(), as otherwise reuse will be unsafe. Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Oleg Nesterov <oleg@redhat.com> Link: https://lore.kernel.org/r/20241206002417.3295533-4-andrii@kernel.org
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