Re: [nf PATCH v2 8/8] netfilter: nf_tables: Add locking for NFT_MSG_GETSETELEM_RESET requests

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On Fri, Sep 29, 2023 at 01:30:43PM +0200, Florian Westphal wrote:
> Phil Sutter <phil@xxxxxx> wrote:
> > On Thu, Sep 28, 2023 at 10:07:51PM +0200, Florian Westphal wrote:
> > > I don't really like it though because misbehaving userspace
> > > can lock out writers.
> > 
> > Make them inactive and free only after the dump is done? IIUC,
> > nft_active_genmask() will return true again though after the second
> > update, right?
> Yes, however, in case of update and 'reset dump', we'll set the
> NLM_F_DUMP_INTR flag, so userspace would restart the dump.
> AFAIU, this means the original values of 'already-reset' counters
> are lost given nft will restart the 'reset dump'.
> Alternative is make nft not restart if reset-dump was requested,
> but in that case the dump can be incomplete.

Modification of the data being dump-reset is unsolvable anyway, unless
we can undo the reset. Not having to return EINTR for unrelated
modifications would help already, though may just be yet another
half-ass solution.

I'd honestly just document the unreliability of 'reset rules' and point
at 'reset rule' for a safe variant. (Assuming the non-dump path is
actually safe?!)

Cheers, Phil

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