Re: [iptables PATCH] nft: Increase BATCH_PAGE_SIZE to support huge rulesets

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

On Fri, Apr 02, 2021 at 07:38:10AM +0200, Florian Westphal wrote:
> Phil Sutter <phil@nwl.cc> wrote:
> > In order to support the same ruleset sizes as legacy iptables, the
> > kernel's limit of 1024 iovecs has to be overcome. Therefore increase
> > each iovec's size from 256KB to 4MB.
> > 
> > While being at it, add a log message for failing sendmsg() call. This is
> > not supposed to happen, even if the transaction fails. Yet if it does,
> > users are left with only a "line XXX failed" message (with line number
> > being the COMMIT line).
> > 
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > ---
> >  iptables/nft.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/iptables/nft.c b/iptables/nft.c
> > index bd840e75f83f4..e19c88ece6c2a 100644
> > --- a/iptables/nft.c
> > +++ b/iptables/nft.c
> > @@ -88,11 +88,11 @@ int mnl_talk(struct nft_handle *h, struct nlmsghdr *nlh,
> >  
> >  #define NFT_NLMSG_MAXSIZE (UINT16_MAX + getpagesize())
> >  
> > -/* selected batch page is 256 Kbytes long to load ruleset of
> > - * half a million rules without hitting -EMSGSIZE due to large
> > - * iovec.
> > +/* Selected batch page is 4 Mbytes long to support loading a ruleset of 3.5M
> > + * rules matching on source and destination address as well as input and output
> > + * interfaces. This is what legacy iptables supports.
> >   */
> > -#define BATCH_PAGE_SIZE getpagesize() * 32
> > +#define BATCH_PAGE_SIZE getpagesize() * 512
> 
> Why not remove getpagesize() altogether?
> 
> The comment assumes getpagesize returns 4096 so might as well just use
> "#define BATCH_PAGE_SIZE  (4 * 1024 * 1024)" or similar?

Agreed.

> On my system getpagesize() * 512 yields 2097152 ...
>
> >  static struct nftnl_batch *mnl_batch_init(void)
> >  {
> > @@ -220,8 +220,10 @@ static int mnl_batch_talk(struct nft_handle *h, int numcmds)
> >  	int err = 0;
> >  
> >  	ret = mnl_nft_socket_sendmsg(h, numcmds);
> > -	if (ret == -1)
> > +	if (ret == -1) {
> > +		fprintf(stderr, "sendmsg() failed: %s\n", strerror(errno));
> >  		return -1;
> > +	}
> 
> Isn't that library code?  At the very least this should use
> nft_print().

I'm not sure this update is required. EMSGSIZE should only come from
sendmsg() in the mnl_batch_talk() path.