Hi, Here is an RFC patch series which removes standard RCU read lock (guard(rcu) and rcu_read_lock()) from fprobe callback paths by switching fprobe and BPF multi-kprobe to Tasks-Rude RCU.
Motivation & Problem ==================== Currently, fprobe entry and exit callbacks (fprobe_fgraph_entry, fprobe_return, and fprobe_ftrace_entry) execute in the tracing hot path where preemption is disabled. However, fprobe was forced to wrap hash lookups with guard(rcu)() and rcu_read_lock() because: - rhashtable defers bucket table deallocation using standard RCU - unregister_fprobe() and BPF waited for a standard RCU grace period Taking standard RCU read locks in the tracing fast path introduces several drawbacks: 1. Unnecessary Runtime Overhead: Every probe hit manipulates current->rcu_read_lock_nesting with memory barriers, and rcu_read_unlock() adds conditional branches to check for special quiescent processing. On debug kernels with CONFIG_PROVE_RCU=y or CONFIG_LOCKDEP=y, this additionally acquires and releases lockdep maps on every hit, introducing severe lockdep hashing overhead and tracer recursion risks. 2. Fragile Dependency on rcu_is_watching(): Standard RCU treats idle CPUs (and user-space on nohz_full CPUs) as Extended Quiescent States (EQS). If a function is traced while rcu_is_watching() is false, standard RCU is blind to the read-side critical section. In such contexts, synchronize_rcu() does not wait for the reader (risking Use-After-Free), and lockdep emits an "RCU-illegal: rcu_read_lock() used while not watching!" warning. To avoid this, ftrace callbacks normally require FTRACE_OPS_FL_RCU, adding extra trampoline check overhead. 3. Asymmetric Synchronization with fprobe_return(): fprobe_return() executes under preempt_disable_notrace() without holding rcu_read_lock(). Prior to this series, unregister_fprobe() only waited on synchronize_rcu(), which does NOT wait for pure preempt-disabled sections, leaving a potential Use-After-Free window during probe unregistration. I've tried to fix the last UAF with simply introducing guard(rcu)()[1] but Sashiko found the 2nd problem [2]. So I decided to implement this series. [1] https://lore.kernel.org/all/179055575009.241711.6358052647499787191.stgit@devnote2/ [2] https://lore.kernel.org/all/[email protected]/ Solution: Tasks-Rude RCU ======================== Because fprobe callbacks already run strictly within preempt-disabled contexts, we can transition fprobe and its deferred table reclamation to Tasks-Rude RCU: - Tasks-Rude RCU detects grace periods via schedule_on_each_cpu(), forcing a schedule on every online CPU. This guarantees that all preempt-disabled sections that began prior to the grace period have completed before memory is reclaimed. - Unlike standard RCU, Tasks-Rude RCU does not rely on dyntick-idle / EQS tracking. It does NOT require rcu_is_watching() to be true and does not trigger lockdep warnings in pre-RCU/idle execution paths. - Within fprobe, guard(rcu)() is replaced with guard(rcu_sched_notrace)(), reducing the lookup lock to pure, non-tracing preempt counter increments without lockdep or RCU state manipulation. Feedback and suggestions from RCU, BPF, and tracing maintainers are welcome! Thank you, --- base-commit: 5bfa9f1a9dcb6ecb607adbc1c0226605c972935b Masami Hiramatsu (Google) (3): rcu/tasks: Export call_rcu_tasks_rude() rhashtable: Add use_tasks_rude parameter to defer bucket table free fprobe: Switch fprobe and BPF kprobe-multi to Tasks-Rude RCU include/linux/rcupdate.h | 6 +++ include/linux/rhashtable-types.h | 2 + kernel/bpf/syscall.c | 2 + kernel/rcu/tasks.h | 8 ++--- kernel/trace/fprobe.c | 66 +++++++++++++++++++++----------------- lib/rhashtable.c | 5 ++- 6 files changed, 54 insertions(+), 35 deletions(-) -- Masami Hiramatsu (Google) <[email protected]>
