xtables: Fix for segfault in iptables-nft

Trying to set a chain's policy in an invalid table resulted in a
segfault. Reproducer was:

| # iptables -t broute -P BROUTING ACCEPT

Fix this by aborting in nft_chain_new() if nft_table_builtin_find()
returned NULL for the given table name.

For an illustrative error message, set errno to ENXIO in the above case
and add an appropriate Mesage to nft_strerror().

While being at it, improve the error message if an invalid policy was
given. Before:

| # iptables-nft -t filter -P INPUT ACCEPTdf
| iptables: Incompatible with this kernel.

After:

| # iptables-nft -t filter -P INPUT ACCEPTdf
| iptables: Bad policy name. Run `dmesg' for more information.

Third unrelated change in this patch: Drop error checking of
nft_chain_set() in do_commandx(): The function never returns negative,
so that check never yielded true.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>

0 files changed, 0 insertions, 0 deletions