On Tue, Sep 29, 2026 at 09:19:12AM +0900, Masami Hiramatsu (Google) wrote: > From: Masami Hiramatsu (Google) <[email protected]> > > unregister_fprobe() and unregister_fprobe_async() (used by BPF > kprobe-multi) rely on standard RCU grace periods (synchronize_rcu() > and call_rcu()) to wait until in-flight fprobe handlers complete before > freeing the fprobe. > > However, if an fprobe handler executes while RCU is not watching (such > as in the idle loop or nohz_full extended quiescent states), standard > RCU does not track preemption-disabled sections. Consequently, > synchronize_rcu() does not wait for those executions, which can lead > to a use-after-free if the fprobe is freed immediately after > unregistration. Ensure handlers exit early when !rcu_is_watching(). > > Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously > used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every > hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep > recursion if probed functions interact with lockdep. > > Since rhltable_lookup() and rhl_for_each_entry_rcu() use > rcu_dereference_all_check() (which checks rcu_read_lock_any_held()), > holding preemption disabled via rcu_read_lock_sched_notrace() is fully > valid and sufficient so long as rcu_is_watching() is true. > > Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(), > fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path > rcu_read_lock() and lockdep overhead while guaranteeing safe grace > period synchronization. > > Reported-by: Sashiko <[email protected]> > Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3 > Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace > period") > Cc: [email protected] > Assisted-by: LLM > Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
>From an RCU perspective: Reviewed-by: Paul E. McKenney <[email protected]> > --- > kernel/trace/fprobe.c | 26 ++++++++++++++++---------- > 1 file changed, 16 insertions(+), 10 deletions(-) > > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c > index 9f2d98181779..da286619c5d8 100644 > --- a/kernel/trace/fprobe.c > +++ b/kernel/trace/fprobe.c > @@ -47,6 +47,10 @@ static struct rhltable fprobe_ip_table; > static DEFINE_MUTEX(fprobe_mutex); > static struct fgraph_ops fprobe_graph_ops; > > +DEFINE_LOCK_GUARD_0(rcu_sched_notrace, > + rcu_read_lock_sched_notrace(), > + rcu_read_unlock_sched_notrace()) > + > static u32 fprobe_node_hashfn(const void *data, u32 len, u32 seed) > { > return hash_ptr(*(unsigned long **)data, 32); > @@ -329,16 +333,14 @@ static void fprobe_ftrace_entry(unsigned long ip, > unsigned long parent_ip, > struct fprobe *fp; > int bit; > > + if (!rcu_is_watching()) > + return; > + > bit = ftrace_test_recursion_trylock(ip, parent_ip); > if (bit < 0) > return; > > - /* > - * ftrace_test_recursion_trylock() disables preemption, but > - * rhltable_lookup() checks whether rcu_read_lcok is held. > - * So we take rcu_read_lock() here. > - */ > - rcu_read_lock(); > + guard(rcu_sched_notrace)(); > head = rhltable_lookup(&fprobe_ip_table, &ip, fprobe_rht_params); > > rhl_for_each_entry_rcu(node, pos, head, hlist) { > @@ -353,7 +355,6 @@ static void fprobe_ftrace_entry(unsigned long ip, > unsigned long parent_ip, > else > __fprobe_handler(ip, parent_ip, fp, fregs, NULL); > } > - rcu_read_unlock(); > ftrace_test_recursion_unlock(bit); > } > NOKPROBE_SYMBOL(fprobe_ftrace_entry); > @@ -567,10 +568,13 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent > *trace, struct fgraph_ops > struct fprobe *fp; > int used, ret; > > + if (!rcu_is_watching()) > + return 0; > + > if (WARN_ON_ONCE(!fregs)) > return 0; > > - guard(rcu)(); > + guard(rcu_sched_notrace)(); > head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params); > reserved_words = 0; > rhl_for_each_entry_rcu(node, pos, head, hlist) { > @@ -665,13 +669,16 @@ static void fprobe_return(struct ftrace_graph_ret > *trace, > int size, curr; > int size_words; > > + if (!rcu_is_watching()) > + return; > + > fgraph_data = (unsigned long *)fgraph_retrieve_data(gops->idx, &size); > if (WARN_ON_ONCE(!fgraph_data)) > return; > size_words = SIZE_IN_LONG(size); > ret_ip = ftrace_regs_get_instruction_pointer(fregs); > > - preempt_disable_notrace(); > + guard(rcu_sched_notrace)(); > > curr = 0; > while (size_words > curr) { > @@ -687,7 +694,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace, > } > curr += size; > } > - preempt_enable_notrace(); > } > NOKPROBE_SYMBOL(fprobe_return); > >
