Re: [PATCH bpf] bpf: fix a rcu usage warning in bpf_prog_array_copy_core()

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Aug 14, 2018 at 11:01:12AM -0700, Yonghong Song wrote:
> Commit 394e40a29788 ("bpf: extend bpf_prog_array to store pointers
> to the cgroup storage") refactored the bpf_prog_array_copy_core()
> to accommodate new structure bpf_prog_array_item which contains
> bpf_prog array itself.
> 
> In the old code, we had
>    perf_event_query_prog_array():
>      mutex_lock(...)
>      bpf_prog_array_copy_call():
>        prog = rcu_dereference_check(array, 1)->progs
>        bpf_prog_array_copy_core(prog, ...)
>      mutex_unlock(...)
> 
> With the above commit, we had
>    perf_event_query_prog_array():
>      mutex_lock(...)
>      bpf_prog_array_copy_call():
>        bpf_prog_array_copy_core(array, ...):
>          item = rcu_dereference(array)->items;
>          ...
>      mutex_unlock(...)
> 
> The new code will trigger a lockdep rcu checking warning.
> The fix is to change rcu_dereference() to rcu_dereference_check()
> to prevent such a warning.
> 
> Reported-by: syzbot+6e72317008eef84a216b@syzkaller.appspotmail.com
> Fixes: 394e40a29788 ("bpf: extend bpf_prog_array to store pointers to the cgroup storage")
> Cc: Roman Gushchin <guro@fb.com>
> Signed-off-by: Yonghong Song <yhs@fb.com>

makes sense to me
Acked-by: Alexei Starovoitov <ast@kernel.org>

Roman, would you agree?