On Tue, Oct 13, 2020 at 12:15:02PM +0200, Pablo Neira Ayuso wrote:
> On Tue, Oct 13, 2020 at 12:08:03PM +0200, Phil Sutter wrote:
> [...]
> > On Mon, Oct 12, 2020 at 02:54:50PM +0200, Pablo Neira Ayuso wrote:
> > [...]
> > > Patch LGTM, thanks Phil.
> > >
> > > What I don't clearly see yet is what scenario is triggering the bug in
> > > the existing code, if you don't mind to explain.
> >
> > See the test case attached to the patch: An other iptables-restore
> > process may add references (i.e., jumps) to a chain the own
> > iptables-restore process wants to delete. This should not be a problem
> > because these references are added to a chain that is being flushed by
> > the own process as well. But if that chain doesn't exist while the own
> > process fetches kernel's ruleset, this flush job is not created.
>
> Let me rephrase this:
>
> 1) process A fetches the ruleset, finds no chain C (no flush job then)
> 2) process B adds new chain C, flush job is present
> 3) process B adds the ruleset
> 4) process A appends rules to the existing chain C (because there is
> no flush job)
>
> Is this the scenario? If so, I wonder why the generation ID is not
> helping to refresh and retry.
Not quite, let me try to put this more clearly:
* Dump A:
| *filter
| :FOO - [0:0] # flush chain FOO
| -X BAR # remove chain BAR
| COMMIT
* Dump B:
| *filter
| -A FOO -j BAR # reference BAR from a rule in FOO
| COMMIT
* Kernel ruleset:
| *filter
| :BAR - [0:0]
| COMMIT
* Process A:
* read dump A
* fetch cache
* Process B:
* read dump B
* fetch ruleset
* commit to kernel
* Process A:
* skip flush chain FOO job: not present
* add delete chain BAR job: chain exists
* commit fails (genid outdated)
* refresh transaction:
* delete chain BAR job remains active
* genid updated
* commit fails: can't remove chain BAR: EBUSY
I realize the test case is not quite effective, ruleset should be
emptied upon each iteration of concurrent restore job startup.
Cheers, Phil
