linux-kernel - [PATCH -tip V2 1/2] kprobes: Suppress the suspicious RCU warning on kprobes

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Date:   Tue,  3 Dec 2019 15:06:17 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Anders Roxell <anders.roxell@...aro.org>, paulmck@...nel.org,
        joel@...lfernandes.org,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        David Miller <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH -tip V2 1/2] kprobes: Suppress the suspicious RCU warning on kprobes

Anders reported that the lockdep warns that suspicious
RCU list usage in register_kprobe() (detected by
CONFIG_PROVE_RCU_LIST.) This is because get_kprobe()
access kprobe_table[] by hlist_for_each_entry_rcu()
without rcu_read_lock.

If we call get_kprobe() from the breakpoint handler context,
it is run with preempt disabled, so this is not a problem.
But in other cases, instead of rcu_read_lock(), we locks
kprobe_mutex so that the kprobe_table[] is not updated.
So, current code is safe, but still not good from the view
point of RCU.

Joel suggested that we can silent that warning by passing
lockdep_is_held() to the last argument of
hlist_for_each_entry_rcu().

Add lockdep_is_held(&kprobe_mutex) at the end of the
hlist_for_each_entry_rcu() to suppress the warning.

Reported-by: Anders Roxell <anders.roxell@...aro.org>
Suggested-by: Joel Fernandes <joel@...lfernandes.org>
Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 kernel/kprobes.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 53534aa258a6..f9ecb6d532fb 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -326,7 +326,8 @@ struct kprobe *get_kprobe(void *addr)
 	struct kprobe *p;

 	head = &kprobe_table[hash_ptr(addr, KPROBE_HASH_BITS)];
-	hlist_for_each_entry_rcu(p, head, hlist) {
+	hlist_for_each_entry_rcu(p, head, hlist,
+				 lockdep_is_held(&kprobe_mutex)) {
 		if (p->addr == addr)
 			return p;
 	}