Re: [nft PATCH] libnftables: Fix for multiple context instances

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Mon, Nov 20, 2017 at 04:58:22PM +0100, Phil Sutter wrote:
> On Mon, Nov 20, 2017 at 02:07:32PM +0100, Pablo Neira Ayuso wrote:
> > On Mon, Nov 20, 2017 at 01:54:18PM +0100, Phil Sutter wrote:
> > > On Mon, Nov 20, 2017 at 01:37:26PM +0100, Pablo Neira Ayuso wrote:
> > > > On Thu, Nov 16, 2017 at 08:10:24PM +0100, Phil Sutter wrote:
> > > > > If a second context is created, the second call to nft_ctx_free() leads
> > > > > to freeing invalid pointers in nft_exit(). Fix this by introducing a
> > > > > reference counter so that neither nft_init() nor nft_exit() run more
> > > > > than once in a row.
> > > > > 
> > > > > This reference counter has to be protected from parallel access. Do this
> > > > > using a mutex in a way that once nft_init() returns, the first call to
> > > > > that function running in parallel is guaranteed to be finished -
> > > > > otherwise it could happen that things being initialized in one thread
> > > > > are already accessed in another one.
> > > > 
> > > > I would prefer this table is placed into the context object, so they
> > > > are not global anymore. So I would prefer we fix this the right way(tm).
> > > > 
> > > > Let me know your thoughts, thanks!
> > > 
> > > I don't think that's feasible: Looking at 'mark_tbl' for instance, this is
> > > accessed from callbacks in 'mark_type', so we would have to make nft_ctx
> > > accessible by all functions dealing with datatypes.
> > 
> > Probably some specific new context object that wrap access to these
> > tables, not necessarily nft_ctx.
> > 
> > It's just more code, it will not be a small patch, but I don't see any
> > reason this can't be done.
> 
> Yes, this is my guess as well. Though for what benefit? I don't think
> having this logic for global run-time data is bad per se. What is it
> that you don't like about it?

This is breaking the assumption that releasing the context object will
be clearing all state behind us.