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+6e72317008eef84a2...@syzkaller.appspotmail.com
> > Fixes: 394e40a29788 ("bpf: extend bpf_prog_array to store pointers to the
> > cgroup storage")
> > Cc: Roman Gushchin <g...@fb.com>
> > Signed-off-by: Yonghong Song <y...@fb.com>
>
> makes sense to me
> Acked-by: Alexei Starovoitov <a...@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.
Thanks!
