Re: [PATCHv2 iproute2] ip route: ignore ENOENT during save if RT_TABLE_MAIN is being dumped

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 24 Jun 2021 18:28:12 +0300
Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com> wrote:

> We started to use in-kernel filtering feature which allows to get only needed
> tables (see iproute_dump_filter()). From the kernel side it's implemented in
> net/ipv4/fib_frontend.c (inet_dump_fib), net/ipv6/ip6_fib.c (inet6_dump_fib).
> The problem here is that behaviour of "ip route save" was changed after
> c7e6371bc ("ip route: Add protocol, table id and device to dump request").
> If filters are used, then kernel returns ENOENT error if requested table is absent,
> but in newly created net namespace even RT_TABLE_MAIN table doesn't exist.
> It is really allocated, for instance, after issuing "ip l set lo up".
> 
> Reproducer is fairly simple:
> $ unshare -n ip route save > dump
> Error: ipv4: FIB table does not exist.
> Dump terminated
> 
> Expected result here is to get empty dump file (as it was before this change).
> 
> v2: reworked, so, now it takes into account NLMSGERR_ATTR_MSG
> (see nl_dump_ext_ack_done() function). We want to suppress error messages
> in stderr about absent FIB table from kernel too.
> 
> Fixes: c7e6371bc ("ip route: Add protocol, table id and device to dump request")
> Cc: David Ahern <dsahern@gmail.com>
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Cc: Andrei Vagin <avagin@gmail.com>
> Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
> Signed-off-by: Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com>
> ---
>  include/libnetlink.h |  5 +++++
>  ip/iproute.c         |  8 +++++++-
>  lib/libnetlink.c     | 31 ++++++++++++++++++++++++++-----
>  3 files changed, 38 insertions(+), 6 deletions(-)
> 
> diff --git a/include/libnetlink.h b/include/libnetlink.h
> index b9073a6a..93c22a09 100644
> --- a/include/libnetlink.h
> +++ b/include/libnetlink.h
> @@ -121,6 +121,11 @@ int rtnl_dump_filter_nc(struct rtnl_handle *rth,
>  			void *arg, __u16 nc_flags);
>  #define rtnl_dump_filter(rth, filter, arg) \
>  	rtnl_dump_filter_nc(rth, filter, arg, 0)
> +int rtnl_dump_filter_suppress_rtnl_errmsg_nc(struct rtnl_handle *rth,
> +		     rtnl_filter_t filter,
> +		     void *arg1, __u16 nc_flags, const int *errnos);
> +#define rtnl_dump_filter_suppress_rtnl_errmsg(rth, filter, arg, errnos) \
> +	rtnl_dump_filter_suppress_rtnl_errmsg_nc(rth, filter, arg, 0, errnos)
>  int rtnl_talk(struct rtnl_handle *rtnl, struct nlmsghdr *n,
>  	      struct nlmsghdr **answer)
>  	__attribute__((warn_unused_result));
> diff --git a/ip/iproute.c b/ip/iproute.c
> index 5853f026..796d6d17 100644
> --- a/ip/iproute.c
> +++ b/ip/iproute.c
> @@ -1734,6 +1734,8 @@ static int iproute_list_flush_or_save(int argc, char **argv, int action)
>  	char *od = NULL;
>  	unsigned int mark = 0;
>  	rtnl_filter_t filter_fn;
> +	/* last 0 is array trailing */
> +	int suppress_rtnl_errnos[2] = { 0, 0 };
>  

The design would be clearer if there were two arguments rather than magic array of size 2.
Also these are being used as boolean.