Re: [PATCH nft 1/3] libnftables: add nft_ctx_set_netns()

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

On Tue, Jan 14, 2020 at 11:25:16AM +0100, Phil Sutter wrote:
> Hi Pablo,
> 
> On Sun, Jan 12, 2020 at 11:40:27AM +0100, Pablo Neira Ayuso wrote:
> > On Sun, Jan 12, 2020 at 11:28:02AM +0100, Pablo Neira Ayuso wrote:
> > > On Fri, Jan 10, 2020 at 01:53:11PM +0100, Phil Sutter wrote:
> > > > On Thu, Jan 09, 2020 at 06:21:13PM +0100, Pablo Neira Ayuso wrote:
> > > > [...]
> > > > > diff --git a/include/nftables/libnftables.h b/include/nftables/libnftables.h
> > > > > index 765b20dd71ee..887628959ac6 100644
> > > > > --- a/include/nftables/libnftables.h
> > > > > +++ b/include/nftables/libnftables.h
> > > > > @@ -34,10 +34,13 @@ enum nft_debug_level {
> > > > >   * Possible flags to pass to nft_ctx_new()
> > > > >   */
> > > > >  #define NFT_CTX_DEFAULT		0
> > > > > +#define NFT_CTX_NETNS		1
> > > > 
> > > > What is this needed for?
> > > 
> > > The socket is initialized from nft_ctx_init(), and such initialization
> > > needs to happen after the netns switch.
> > 
> > s/nft_ctx_init()/nft_ctx_new()
> 
> Ah, I missed that socket init in nft_ctx_new() happens only if flags is
> zero.
> 
> > > > >  struct nft_ctx *nft_ctx_new(uint32_t flags);
> > > > >  void nft_ctx_free(struct nft_ctx *ctx);
> > > > >  
> > > > > +int nft_ctx_set_netns(struct nft_ctx *ctx, const char *netns);
> > > > 
> > > > Is there a way to select init ns again?
> > > 
> > > AFAIK, setns() does not let you go back to init ns once set.
> 
> I noticed something I find worse, namely that libnftables as a library
> changes the application's netns. Anything it does after changing the
> context's netns applies to that netns only, no matter if it's creating a
> new nft context with NFT_CtX_DEFAULT flag or call iproute via system().
> 
> If we can't find a way to exit the netns again, one can safely assume
> that we are trapping a user's application in a netns with this feature.

IIRC, you can fork(), then let the child enter the netns while parent
remain in the original netns.

> Maybe we should restrict per-netns operation to nft utility and perform
> the netns switch there? Maybe we could provide a "switch_netns()"
> routine in libnftables which is not bound to nft context so users may
> use it in their application?

That's another possibility, yes. In that case, there is no need for
NFT_CTX_NETNS, which is just there to skip the socket initialization.