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    Subject[PATCH 5.4 135/178] kprobes: Suppress the suspicious RCU warning on kprobes
    Date
    From: Masami Hiramatsu <mhiramat@kernel.org>

    [ Upstream commit 6743ad432ec92e680cd0d9db86cb17b949cf5a43 ]

    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.

    Link: http://lkml.kernel.org/r/158927055350.27680.10261450713467997503.stgit@devnote2

    Reported-by: Anders Roxell <anders.roxell@linaro.org>
    Suggested-by: Joel Fernandes <joel@joelfernandes.org>
    Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/kprobes.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/kernel/kprobes.c b/kernel/kprobes.c
    index 195ecb955fcc5..950a5cfd262ce 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;
    }
    --
    2.25.1
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