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Subject[PATCH net-next v1 0/5] rtnetlink: add IFA_IF_NETNSID for RTM_GETADDR

# v1 introduction:
The only functional change is the export of rtnl_get_net_ns_capable()
which is needed in case ipv6 is built as a module.

Note, I did not change the property name to IFA_TARGET_NSID as there was
no clear agreement what would be preferred. My personal preference is to
keep the IFA_IF_NETNSID name because it aligns naturally with the
IFLA_IF_NETNSID property for RTM_*LINK requests. Jiri seems to prefer
this name too.
However, if there is agreement that another property name makes more
sense I'm happy to send a v2 that changes this.

## Performance:
To test this patchset I performed 1 million getifaddrs() requests
against a network namespace containing 5 interfaces (lo, eth{0-4}). The
first test used a network namespace aware getifaddrs() implementation I
wrote and the second test used the traditional setns() + getifaddrs()
method. The results show that this patchsets allows userspace to cut
retrieval time in half:
1. netns_getifaddrs(): 82 microseconds
2. setns() + getifaddrs(): 162 microseconds

# v0 introduction:
A while back we introduced and enabled IFLA_IF_NETNSID in
RTM_{DEL,GET,NEW}LINK requests (cf. [1], [2], [3], [4], [5]). This has led
to signficant performance increases since it allows userspace to avoid
taking the hit of a setns(netns_fd, CLONE_NEWNET), then getting the
interfaces from the netns associated with the netns_fd. Especially when a
lot of network namespaces are in use, using setns() becomes increasingly
problematic when performance matters.
Usually, RTML_GETLINK requests are followed by RTM_GETADDR requests (cf.
getifaddrs() style functions and friends). But currently, RTM_GETADDR
requests do not support a similar property like IFLA_IF_NETNSID for
RTM_*LINK requests.
This is problematic since userspace can retrieve interfaces from another
network namespace by sending a IFLA_IF_NETNSID property along but
RTM_GETLINK request but is still forced to use the legacy setns() style of
retrieving interfaces in RTM_GETADDR requests.

The goal of this series is to make it possible to perform RTM_GETADDR
requests on different network namespaces. To this end a new IFA_IF_NETNSID
property for RTM_*ADDR requests is introduced. It can be used to send a
network namespace identifier along in RTM_*ADDR requests. The network
namespace identifier will be used to retrieve the target network namespace
in which the request is supposed to be fulfilled. This aligns the behavior
of RTM_*ADDR requests with the behavior of RTM_*LINK requests.

## Security:
- The caller must have assigned a valid network namespace identifier for
the target network namespace.
- The caller must have CAP_NET_ADMIN in the owning user namespace of the
target network namespace.


[1]: commit 7973bfd8758d ("rtnetlink: remove check for IFLA_IF_NETNSID")
[2]: commit 5bb8ed075428 ("rtnetlink: enable IFLA_IF_NETNSID for RTM_NEWLINK")
[3]: commit b61ad68a9fe8 ("rtnetlink: enable IFLA_IF_NETNSID for RTM_DELLINK")
[4]: commit c310bfcb6e1b ("rtnetlink: enable IFLA_IF_NETNSID for RTM_SETLINK")
[5]: commit 7c4f63ba8243 ("rtnetlink: enable IFLA_IF_NETNSID in do_setlink()")

Christian Brauner (5):
rtnetlink: add rtnl_get_net_ns_capable()
if_addr: add IFA_IF_NETNSID
rtnetlink: move type calculation out of loop

include/net/rtnetlink.h | 1 +
include/uapi/linux/if_addr.h | 1 +
net/core/rtnetlink.c | 19 +++++++---
net/ipv4/devinet.c | 38 +++++++++++++++-----
net/ipv6/addrconf.c | 70 ++++++++++++++++++++++++++++--------
5 files changed, 101 insertions(+), 28 deletions(-)


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