Re: [PATCH 12/14] net: sched: retry action check-insert on concurrent modification

[Jiri Pirko]

Wed, May 16, 2018 at 03:52:20PM CEST, vladbu@xxxxxxxxxxxx wrote:
>
>On Wed 16 May 2018 at 13:21, Jiri Pirko <jiri@xxxxxxxxxxx> wrote:
>> Wed, May 16, 2018 at 02:43:58PM CEST, vladbu@xxxxxxxxxxxx wrote:
>>>
>>>On Wed 16 May 2018 at 12:26, Jiri Pirko <jiri@xxxxxxxxxxx> wrote:
>>>> Wed, May 16, 2018 at 01:55:06PM CEST, vladbu@xxxxxxxxxxxx wrote:
>>>>>
>>>>>On Wed 16 May 2018 at 09:59, Jiri Pirko <jiri@xxxxxxxxxxx> wrote:
>>>>>> Mon, May 14, 2018 at 04:27:13PM CEST, vladbu@xxxxxxxxxxxx wrote:
>>>>>>>Retry check-insert sequence in action init functions if action with same
>>>>>>>index was inserted concurrently.
>>>>>>>
>>>>>>>Signed-off-by: Vlad Buslov <vladbu@xxxxxxxxxxxx>
>>>>>>>---
>>>>>>> net/sched/act_bpf.c        | 8 +++++++-
>>>>>>> net/sched/act_connmark.c   | 8 +++++++-
>>>>>>> net/sched/act_csum.c       | 8 +++++++-
>>>>>>> net/sched/act_gact.c       | 8 +++++++-
>>>>>>> net/sched/act_ife.c        | 8 +++++++-
>>>>>>> net/sched/act_ipt.c        | 8 +++++++-
>>>>>>> net/sched/act_mirred.c     | 8 +++++++-
>>>>>>> net/sched/act_nat.c        | 8 +++++++-
>>>>>>> net/sched/act_pedit.c      | 8 +++++++-
>>>>>>> net/sched/act_police.c     | 9 ++++++++-
>>>>>>> net/sched/act_sample.c     | 8 +++++++-
>>>>>>> net/sched/act_simple.c     | 9 ++++++++-
>>>>>>> net/sched/act_skbedit.c    | 8 +++++++-
>>>>>>> net/sched/act_skbmod.c     | 8 +++++++-
>>>>>>> net/sched/act_tunnel_key.c | 9 ++++++++-
>>>>>>> net/sched/act_vlan.c       | 9 ++++++++-
>>>>>>> 16 files changed, 116 insertions(+), 16 deletions(-)
>>>>>>>
>>>>>>>diff --git a/net/sched/act_bpf.c b/net/sched/act_bpf.c
>>>>>>>index 5554bf7..7e20fdc 100644
>>>>>>>--- a/net/sched/act_bpf.c
>>>>>>>+++ b/net/sched/act_bpf.c
>>>>>>>@@ -299,10 +299,16 @@ static int tcf_bpf_init(struct net *net, struct nlattr *nla,
>>>>>>> 
>>>>>>> 	parm = nla_data(tb[TCA_ACT_BPF_PARMS]);
>>>>>>> 
>>>>>>>+replay:
>>>>>>> 	if (!tcf_idr_check(tn, parm->index, act, bind)) {
>>>>>>> 		ret = tcf_idr_create(tn, parm->index, est, act,
>>>>>>> 				     &act_bpf_ops, bind, true);
>>>>>>>-		if (ret < 0)
>>>>>>>+		/* Action with specified index was created concurrently.
>>>>>>>+		 * Check again.
>>>>>>>+		 */
>>>>>>>+		if (parm->index && ret == -ENOSPC)
>>>>>>>+			goto replay;
>>>>>>>+		else if (ret)
>>>>>>
>>>>>> Hmm, looks like you are doing the same/very similar thing in every act
>>>>>> code. I think it would make sense to introduce a helper function for
>>>>>> this purpose.
>>>>>
>>>>>This code uses goto so it can't be easily refactored into standalone
>>>>>function. Could you specify which part of this code you suggest to
>>>>>extract?
>>>>
>>>> Hmm, looking at the code, I think that what would help is to have a
>>>> helper that would atomically check if index exists and if not, it would
>>>> allocate one. Something like:
>>>>
>>>>
>>>> int tcf_idr_check_alloc(struct tc_action_net *tn, u32 *index,
>>>> 			struct tc_action **a, int bind)
>>>> {
>>>> 	struct tcf_idrinfo *idrinfo = tn->idrinfo;
>>>> 	struct tc_action *p;
>>>> 	int err;
>>>>
>>>> 	spin_lock(&idrinfo->lock);
>>>> 	if (*index) {
>>>> 		p = idr_find(&idrinfo->action_idr, *index);
>>>> 		if (p) {
>>>> 			if (bind)
>>>> 	   			p->tcfa_bindcnt++;
>>>> 			p->tcfa_refcnt++;
>>>> 			*a = p;
>>>> 			err = 0;
>>>> 		} else {
>>>> 			*a = NULL;
>>>> 			err = idr_alloc_u32(idr, NULL, index,
>>>> 					    *index, GFP_ATOMIC);
>>>> 		}
>>>> 	} else {
>>>> 		*index = 1;
>>>> 		*a = NULL;
>>>> 		err = idr_alloc_u32(idr, NULL, index, UINT_MAX, GFP_ATOMIC);
>>>> 	}
>>>> 	spin_unlock(&idrinfo->lock);
>>>> 	return err;
>>>> }
>>>>
>>>> The act code would just check if "a" is NULL and if so, it would call
>>>> tcf_idr_create() with allocated index as arg.
>>>
>>>What about multiple actions that have arbitrary code between initial
>>>check and idr allocation that is currently inside tcf_idr_create()?
>>
>> Why it would be a problem to have them after the allocation?
>
>Lets look at mirred for exmple:
>	exists = tcf_idr_check(tn, parm->index, a, bind);
>	if (exists && bind)
>		return 0;
>
>	switch (parm->eaction) {
>	case TCA_EGRESS_MIRROR:
>	case TCA_EGRESS_REDIR:
>	case TCA_INGRESS_REDIR:
>	case TCA_INGRESS_MIRROR:
>		break;
>	default:
>		if (exists)
>			tcf_idr_release(*a, bind);
>		NL_SET_ERR_MSG_MOD(extack, "Unknown mirred option");
>		return -EINVAL;
>	}
>	if (parm->ifindex) {
>		dev = __dev_get_by_index(net, parm->ifindex);
>		if (dev == NULL) {
>			if (exists)
>				tcf_idr_release(*a, bind);
>			return -ENODEV;
>		}
>		mac_header_xmit = dev_is_mac_header_xmit(dev);
>	} else {
>		dev = NULL;
>	}
>
>	if (!exists) {
>		if (!dev) {
>			NL_SET_ERR_MSG_MOD(extack, "Specified device does not exist");
>			return -EINVAL;
>		}
>		ret = tcf_idr_create(tn, parm->index, est, a,
>				     &act_mirred_ops, bind, true);
>		/* Action with specified index was created concurrently.
>		 * Check again.
>		 */
>		if (parm->index && ret == -ENOSPC)
>			goto replay;
>		else if (ret)
>			return ret;
>
>There are several returns and cleanup is only performed when action
>exists. So all code like that will have to be audited to also remove
>index from idr, otherwise idr handles leak on return.

Yeah. You have to take care of the error path.


>
>>
>> There is one issue though with my draft. tcf_idr_insert() function
>> which actually assigns a "p" pointer to the idr index is called later on.
>> Until that happens, the idr_find() would return NULL even if the index
>> is actually allocated. We cannot assign "p" in tcf_idr_check_alloc()
>> because it is allocated only later on in tcf_idr_create(). But that is
>> resolvable by the following trick:
>>
>> int tcf_idr_check_alloc(struct tc_action_net *tn, u32 *index,
>> 			struct tc_action **a, int bind)
>> {
>> 	struct tcf_idrinfo *idrinfo = tn->idrinfo;
>> 	struct tc_action *p;
>> 	int err;
>>
>> again:
>> 	spin_lock(&idrinfo->lock);
>> 	if (*index) {
>>  		p = idr_find(&idrinfo->action_idr, *index);
>> 		if (IS_ERR(p)) {
>> 			/* This means that another process allocated
>> 			 * index but did not assign the pointer yet.
>> 			 */
>> 			spin_unlock(&idrinfo->lock);
>> 			goto again;
>> 		}
>>  		if (p) {
>>  			if (bind)
>>  	   			p->tcfa_bindcnt++;
>>  			p->tcfa_refcnt++;
>>  			*a = p;
>>  			err = 0;
>>  		} else {
>>  			*a = NULL;
>>  			err = idr_alloc_u32(idr, NULL, index,
>>  					    *index, GFP_ATOMIC);
>> 			idr_replace(&idrinfo->action_idr,
>> 				    ERR_PTR(-EBUSY), *index);
>>  		}
>>  	} else {
>>  		*index = 1;
>> 		*a = NULL;
>>  		err = idr_alloc_u32(idr, NULL, index, UINT_MAX, GFP_ATOMIC);
>> 		idr_replace(&idrinfo->action_idr, ERR_PTR(-EBUSY), *index);
>>  	}
>>  	spin_unlock(&idrinfo->lock);
>>  	return err;
>> }
>>
>
>So users of action idr that might perform concurrent lookups are also
>have to be changed to check for error pointers, that now can be inserted
>into idr? Seems like a complex change...

You can just add a simple check into tcf_idr_lookup(). Where else?


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