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Transaction refresh was broken with regards to nft_chain_restore(): It
created a rule flush batch object only if the chain was found in cache
and a chain add object only if the chain was not found. Yet with
concurrent ruleset updates, one has to expect both situations:
* If a chain vanishes, the rule flush job must be skipped and instead
the chain add job become active.
* If a chain appears, the chain add job must be skipped and instead
rules flushed.
Change the code accordingly: Create both batch objects and set their
'skip' field depending on the situation in cache and adjust both in
nft_refresh_transaction().
As a side-effect, the implicit rule flush becomes explicit and all
handling of implicit batch jobs is dropped along with the related field
indicating such.
Reuse the 'implicit' parameter of __nft_rule_flush() to control the
initial 'skip' field value instead.
A subtle caveat is vanishing of existing chains: Creating the chain add
job based on the chain in cache causes a netlink message containing that
chain's handle which the kernel dislikes. Therefore unset the chain's
handle in that case.
Fixes: 58d7de0181f61 ("xtables: handle concurrent ruleset modifications")
Signed-off-by: Phil Sutter <phil@nwl.cc>
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