Re: [dpdk-dev] [PATCH] lpm: unchecked return value

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Jun 23, 2016 at 09:13:22AM +0100, Jastrzebski, MichalX K wrote:
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Azarewicz, PiotrX T
> > Sent: Thursday, May 12, 2016 1:20 PM
> > To: Mrozowicz, SlawomirX <slawomirx.mrozowicz@intel.com>; Richardson,
> > Bruce <bruce.richardson@intel.com>
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH] lpm: unchecked return value
> > 
> > Hi,
> > 
> > I handle Coverity defect ID 13201. It is about unchecked return value from
> > rte_lpm6_delete() instances in rte_lpm6_add() function.
> > Next I found this thread and I see that both defects (ID 13205 and ID 13201)
> > may be resolved all together.
> > 
> > > >> Fix issue reported by Coverity.
> > > >>
> > > >> Coverity ID 13205: Unchecked return value Unchecked return value
> > > >> check_return: Calling rte_lpm6_add without checking return value
> > > >> Fixes: 5c510e13a9cb ("lpm: add IPv6 support")
> > > >>
> > > >> Signed-off-by: Slawomir Mrozowicz <slawomirx.mrozowicz@intel.com>
> > > >> ---
> > > >>  lib/librte_lpm/rte_lpm6.c | 10 ++++++----
> > > >>  1 file changed, 6 insertions(+), 4 deletions(-)
> > > >>
> > > >> diff --git a/lib/librte_lpm/rte_lpm6.c b/lib/librte_lpm/rte_lpm6.c
> > > >> index ba4353c..f4db3fa 100644
> > > >> --- a/lib/librte_lpm/rte_lpm6.c
> > > >> +++ b/lib/librte_lpm/rte_lpm6.c
> > > >> @@ -749,6 +749,7 @@ rte_lpm6_delete(struct rte_lpm6 *lpm, uint8_t
> > > >> *ip,
> > > >uint8_t depth)
> > > >>  	int32_t rule_to_delete_index;
> > > >>  	uint8_t ip_masked[RTE_LPM6_IPV6_ADDR_SIZE];
> > > >>  	unsigned i;
> > > >> +	int status = 0;
> > > >>
> > > >>  	/*
> > > >>  	 * Check input arguments.
> > > >> @@ -790,12 +791,13 @@ rte_lpm6_delete(struct rte_lpm6 *lpm,
> > uint8_t
> > > >*ip, uint8_t depth)
> > > >>  	 * Add every rule again (except for the one that was removed from
> > > >>  	 * the rules table).
> > > >>  	 */
> > > >> -	for (i = 0; i < lpm->used_rules; i++) {
> > > >> -		rte_lpm6_add(lpm, lpm->rules_tbl[i].ip, lpm-
> > > >>rules_tbl[i].depth,
> > > >> -				lpm->rules_tbl[i].next_hop);
> > > >> +	for (i = 0; i < lpm->used_rules && status >= 0; i++) {
> > > >> +		status = rte_lpm6_add(
> > > >> +			lpm, lpm->rules_tbl[i].ip, lpm->rules_tbl[i].depth,
> > > >> +			lpm->rules_tbl[i].next_hop);
> > > >>  	}
> > > >>
> > > >> -	return 0;
> > > >> +	return status;
> > > >>  }
> > > >
> > > >Hi,
> > > >
> > > >I'm not sure that this patch is actually necessary, as I'm not sure
> > > >that the lpm6_add calls can fail in this instance. Looking through the
> > > >code, this function deletes the rule and then clears the actual lpm
> > > >lookup tables before re-adding all other routes to it again. The only
> > > >error condition that could be returned, that I can see, is -ENOSPC,
> > > >which should never occur here since the original rules fitted in the first
> > > place.
> > 
> > I agree that -ENOSPC should never occur here. So rte_lpm6_add() instance
> > should never fail here.
> > 
> > Next I looked at rte_lpm6_add() and if rte_lpm6_delete() instances in it
> > may fail?
> > The only suspicious place that I found is place when add every rule again
> > but that should work as discussed above.
> > 
> > > >
> > > >If it was possible to fail, then I think we would have a worse problem,
> > > >in that deleting a single rule has wiped out our lpm table and left it
> > > >in an inconsistent state, so the error handling probably needs to be
> > better
> > > than just quitting.
> > > >
> > > >Finally, one other thing I spot looking through the code, is that there
> > > >seems to be a worrying set of calls between add and delete. If the add
> > > >function fails, then it calls delete which in turn will call add again,
> > > >etc. etc. This may all work correctly, but it seems fragile and error
> > > >prone to me - especially if we allow calls from one to another to fail.
> > > >
> > > >This looks like it might need some further examination to verify what
> > > >the possible failure cases are and what happens in each scenario.
> > 
> > I see no failure scenarios in here. I mean I see no possibility to create test
> > that show that add function fail in del and opposite.
> > The only scenario what I have in my mind is that someone call add or/and
> > del functions on different threads with the same lpm table instance, but
> > this is not allowed, cause we know that this functions are not thread safe.
> > 
> > > >
> > > >Regards,
> > > >/Bruce
> > >
> > >
> > > Hi Bruce,
> > >
> > > In my opinion the worst-case scenario should be take into account. If
> > > function like rte_lpm6_add() returns false then it should be handled.
> > >
> > > Anyway I agree with you that if the function fail then we have serious
> > > problem.
> > > I see two problems:
> > > 1. Code construction: calls between function rte_lpm6_add() and
> > > rte_lpm6_delete(). As you said it should be examined.
> > > 2. How we should handle situation if the rules table are not reconstructed
> > > after delete operation.
> > >
> > > I propose to add new issue in ClearQuest to proceed solve the problems
> > > because there are extend the original issue (CID 13205 Unchecked return
> > > value) from Coverity.
> > >
> > > Regards,
> > > Sławomir
> > 
> > I propose to classify this Coverity issues (ID 13205 and ID 13201) as
> > Intentional.
> > 
> > Regards,
> > Piotr
> 
> Hi Bruce,
> We would like to move forward with theses Coverity defects thus
> Please share your opinion about classifying these defects as Intentional?
> 
> Michal

>From previous analysis detailed above, it looks like there is no issue with
failing to check the return values here, so I'm ok with this classification.

/Bruce