Subject: Re: [patch net-next 3/6] net_sched: cls_bpf: remove faulty use of list_for_each_entry_rcu

From: Pablo Neira Ayuso <[email-masked]>
Date: Wed, 3 Dec 2014 14:50:48 +0100

On Wed, Dec 03, 2014 at 02:21:42PM +0100, Jiri Pirko wrote:
> Wed, Dec 03, 2014 at 01:19:49PM CET, [email-masked] wrote:
> >On Tue, Dec 02, 2014 at 06:00:33PM +0100, Jiri Pirko wrote:
> >> rcu variant is not correct here. The code is called by updater (rtnl
> >> lock is held), not by reader (no rcu_read_lock is held).
> >> 
> >> Signed-off-by: Jiri Pirko <[email-masked]>
> >> ---
> >>  net/sched/cls_bpf.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/net/sched/cls_bpf.c b/net/sched/cls_bpf.c
> >> index cbfaf6f..d0de979 100644
> >> --- a/net/sched/cls_bpf.c
> >> +++ b/net/sched/cls_bpf.c
> >> @@ -141,7 +141,7 @@ static unsigned long cls_bpf_get(struct tcf_proto *tp, u32 handle)
> >>  	if (head == NULL)
> >>  		return 0UL;
> >>  
> >> -	list_for_each_entry_rcu(prog, &head->plist, link) {
> >> +	list_for_each_entry(prog, &head->plist, link) {
> >>  		if (prog->handle == handle) {
> >>  			ret = (unsigned long) prog;
> >>  			break;
> >> @@ -337,7 +337,7 @@ static void cls_bpf_walk(struct tcf_proto *tp, struct tcf_walker *arg)
> >>  	struct cls_bpf_head *head = rtnl_dereference(tp->root);
> >>  	struct cls_bpf_prog *prog;
> >>  
> >> -	list_for_each_entry_rcu(prog, &head->plist, link) {
> >> +	list_for_each_entry(prog, &head->plist, link) {
> >
> >We still need the _rcu here in the walk path. IIRC, this is called from the
> >dump path and we hold no rtnl_lock there.
> 
> It is called from tc_dump_tfilter only. And tc_dump_tfilter is always
> called with rtnl_lock

Right, nlk->cb_mutex is set to rtnl_lock so netlink_dump() grabs it
for each recvmsg() invocation.