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 05:08:44PM -0700, Roman Gushchin wrote:
> On Tue, Aug 14, 2018 at 04:59:45PM -0700, Alexei Starovoitov wrote:
> > 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?
> > 
> 
> rcu_dereference_check(<>, 1) always looks a bit strange to me,
> but if it's the only reasonable way to silence the warning,
> of course I'm fine with it.

do you have better suggestion?
This patch is a fix for the regression introduced in your earlier patch,
so I think the only fair path forward is either to Ack it or
to send an alternative patch asap.