Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG_DONE into same read() as families

On Tue, Mar 19, 2024 at 08:55:45AM -0700, Jakub Kicinski wrote:

On Fri, 15 Mar 2024 12:48:08 +0100 Stefano Brivio wrote:

quoted

quoted

Make sure ctrl_fill_info() returns sensible error codes and
propagate them out to netlink core. Let netlink core decide
when to return skb->len and when to treat the exit as an
error. Netlink core does better job at it, if we always
return skb->len the core doesn't know when we're done
dumping and NLMSG_DONE ends up in a separate read().  

While this change is obviously correct, it breaks... well, broken
applications that _wrongly_ rely on the fact that NLMSG_DONE is
delivered in a separate datagram.

This was the (embarrassing) case for passt(1), which I just fixed:
  https://archives.passt.top/passt-dev/20240315112432.382212-1-sbrivio@redhat.com/

but the "separate" NLMSG_DONE is such an established behaviour,
I think, that this might raise a more general concern.

From my perspective, I'm just happy that this change revealed the
issue, but I wanted to report this anyway in case somebody has
similar possible breakages in mind.

Hi Stefano! I was worried this may happen :( I think we should revert
offending commits, but I'd like to take it on case by case basis. 
I'd imagine majority of netlink is only exercised by iproute2 and
libmnl-based tools. Does passt hang specifically on genetlink family
dump? Your commit also mentions RTM_GETROUTE. This is not the only
commit which removed DONE:

I don't think there's anything specirfic to RTM_GETROUTE here from the
kernel side.  We've looked at the problem in passt more closely now,
and it turns out we handled a merged NLMSG_DONE correctly in most
cases.  For various reasons internal to passt, our handling of
RTM_GETROUTE on one path is more complex, and we had a subtle error
there which broke the handling of a merged NLMSG_DONE.

$ git log --since='1 month ago' --grep=NLMSG_DONE --no-merges  --oneline 

9cc4cc329d30 ipv6: use xa_array iterator to implement inet6_dump_addr()
87d381973e49 genetlink: fit NLMSG_DONE into same read() as families
4ce5dc9316de inet: switch inet_dump_fib() to RCU protection
6647b338fc5c netlink: fix netlink_diag_dump() return value

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