Re: [PATCHv5 iproute2] ip route: ignore ENOENT during save if RT_TABLE_MAIN is being dumped

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed,  7 Jul 2021 15:22:01 +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.
> 
> v3: reworked to make code clearer. Introduced rtnl_suppressed_errors(),
> rtnl_suppress_error() helpers. User may suppress up to 3 errors (may be
> easily extended by changing SUPPRESS_ERRORS_INIT macro).
> 
> v4: reworked, rtnl_dump_filter_errhndlr() was introduced. Thanks
> to Stephen Hemminger for comments and suggestions
> 
> v5: space fixes, commit message reformat, empty initializers
> 
> 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>

Applied this version