On 08/02/2024 21:51, Willem de Bruijn wrote:

Andy Lutomirski wrote:

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On Thu, Feb 8, 2024 at 11:55 AM Vadim Fedorenko
[off-list ref] wrote:

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On 08/02/2024 18:02, Andy Lutomirski wrote:

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I’ve been using OPT_ID-style timestamping for years, but for some
reason this issue only bit me last week: if sendmsg() fails on a UDP
or ping socket, sk_tskey is poorly.  It may or may not get incremented
by the failed sendmsg().

The intent is indeed to only increment on a successful send.

The implementation is complicated a bit by (1) being a socket level
option, thus also supporting SOCK_RAW and (2) MSG_MORE using multiple
send calls to only produce a single datagram and (3) fragmentation
producing multiple skbs for a single datagram.

If only SOCK_DGRAM, conceivably we could move this to udp_send_skb,
after the skb is created and after the usual error exit paths.

An alternative is in error paths to decrement the counter. This is
what we do for MSG_ZEROCOPY references. Unfortunately, with the
lockless UDP path, other threads could come inbetween the inc and dec,
so this is not really workable.

As I've mentioned before, parallelism with OPT_ID is unpredictable by
design, I don't believe we have real apps doing this, so I think it's
better to decrement sk_tskey to have consistent behavior. I can make the
patch to do it.

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Well, there are several error paths, for sure. For the sockets you
mention the increment of tskey happens at __ip{,6}_append_data. There
are 2 different types of failures which can happen after the increment.
The first is MTU check fail, another one is memory allocation failures.
I believe we can move increment to a later position, after MTU check in
both functions to avoid first type of problem.

For reasons that I still haven't deciphered, I'm sporadically getting
EHOSTUNREACH after the increment.  I can't find anything in the code
that would cause that, and every time I try to instrument it, it stops
happening :(  I sendmsg to the same destination several times in rapid
succession, and at most one of them will get EHOSTUNREACH.

UDP might fail on ICMP responses. Try sending to a closed port. A few
send calls will succeed, but eventually the send call will refuse to
send. The cause is in the IP layer.

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I can think of at least three ways to improve this:

1. Make it so that the sequence number is genuinely only incremented
on success. This may be tedious to implement and may be nearly
impossible if there are multiple concurrent sendmsg() calls on the
same socket.

Multiple concurrent sendmsg() should bring a lot of problems on user-
space side. With current implementation the application has to track the
value of tskey to check incoming TX timestamp later. But with parallel
sendmsg() the app cannot be sure which value is assigned to which call
even in case of proper track value synchronization. That brings us to
the other solutions if we consider having parallel threads working with
same socket. Or we can simply pretend that it's impossible and then fix
error path to decrement tskey value.

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2. Allow the user program to specify an explicit ID.  cmsg values are
variable length, so for datagram sockets, extending the
SO_TIMESTAMPING cmsg with 64 bits of sequence number to be used for
the TX timestamp on that particular packet might be a nice solution.

This option can be really useful in case of really parallel work with
sockets.

I personally like this one the best.  Some care would be needed to
allow programs to detect the new functionality.  Any preferred way to
handle it?

Regardless of whether we can fix the existing behavior, I also think
this is a worthwhile cmsg. As timestamping is a SOL_SOCKET option, the
cmsg should likely also be that, processed in __sock_cmsg_send.

Do you think about extending inet_cork and sockcm_cookie to provide
OPT_ID value? If yes, I can give it a try also.