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Jordan Rife says:
====================
bpf: tcp: Exactly-once socket iteration
TCP socket iterators use iter->offset to track progress through a
bucket, which is a measure of the number of matching sockets from the
current bucket that have been seen or processed by the iterator. On
subsequent iterations, if the current bucket has unprocessed items, we
skip at least iter->offset matching items in the bucket before adding
any remaining items to the next batch. However, iter->offset isn't
always an accurate measure of "things already seen" when the underlying
bucket changes between reads, which can lead to repeated or skipped
sockets. Instead, this series remembers the cookies of the sockets we
haven't seen yet in the current bucket and resumes from the first cookie
in that list that we can find on the next iteration.
This is a continuation of the work started in [1]. This series largely
replicates the patterns applied to UDP socket iterators, applying them
instead to TCP socket iterators.
CHANGES
=======
v5 -> v6:
* In patch ten ("selftests/bpf: Create established sockets in socket
iterator tests"), use poll() to choose a socket that has a connection
ready to be accept()ed. Before, connect_to_server would set the
O_NONBLOCK flag on all listening sockets so that accept_from_one could
loop through them all and find the one that connect_to_addr_str
connected to. However, this is subtly buggy and could potentially lead
to test flakes, since the 3 way handshake isn't necessarily done when
connect returns, so it's possible none of the accept() calls succeed.
Use poll() instead to guarantee that the socket we accept() from is
ready and eliminate the need for the O_NONBLOCK flag (Martin).
v4 -> v5:
* Move WARN_ON_ONCE before the `done` label in patch two ("bpf: tcp:
Make sure iter->batch always contains a full bucket snapshot"")
(Martin).
* Remove unnecessary kfunc declaration in patch eleven ("selftests/bpf:
Create iter_tcp_destroy test program") (Martin).
* Make sure to close the socket fd at the end of `destroy` in patch
twelve ("selftests/bpf: Add tests for bucket resume logic in
established sockets") (Martin).
v3 -> v4:
* Drop braces around sk_nulls_for_each_from in patch five ("bpf: tcp:
Avoid socket skips and repeats during iteration") (Stanislav).
* Add a break after the TCP_SEQ_STATE_ESTABLISHED case in patch five
(Stanislav).
* Add an `if (sock_type == SOCK_STREAM)` check before assigning
TCP_LISTEN to skel->rodata->ss in patch eight ("selftests/bpf: Allow
for iteration over multiple states") to more clearly express the
intent that the option is only consumed for SOCK_STREAM tests
(Stanislav).
* Move the `i = 0` assignment into the for loop in patch ten
("selftests/bpf: Create established sockets in socket iterator
tests") (Stanislav).
v2 -> v3:
* Unroll the loop inside bpf_iter_tcp_batch to make the logic easier to
follow in patch two ("bpf: tcp: Make sure iter->batch always contains
a full bucket snapshot"). This gets rid of the `resizes` variable from
v2 and eliminates the extra conditional that checks how many batch
resize attempts have occurred so far (Stanislav).
Note: This changes the behavior slightly. Before, in the case that
the second call to tcp_seek_last_pos (and later bpf_iter_tcp_resume)
advances to a new bucket, which may happen if the current bucket is
emptied after releasing its lock, the `resizes` "budget" would be
reset, the net effect being that we would try a batch resize with
GFP_USER at most once per bucket. Now, we try to resize the batch
with GFP_USER at most once per call, so it makes it slightly more
likely that we hit the GFP_NOWAIT scenario. However, this edge case
should be rare in practice anyway, and the new behavior is more or
less consistent with the original retry logic, so avoid the loop and
prefer code clarity.
* Move the call to bpf_iter_tcp_put_batch out of
bpf_iter_tcp_realloc_batch and call it directly before invoking
bpf_iter_tcp_realloc_batch with GFP_USER inside bpf_iter_tcp_batch.
/Don't/ call it before invoking bpf_iter_tcp_realloc_batch the second
time while we hold the lock with GFP_NOWAIT. This avoids a conditional
inside bpf_iter_tcp_realloc_batch from v2 that only calls
bpf_iter_tcp_put_batch if flags != GFP_NOWAIT and is a bit more
explicit (Stanislav).
* Adjust patch five ("bpf: tcp: Avoid socket skips and repeats during
iteration") to fit with the new logic in patch two.
v1 -> v2:
* In patch five ("bpf: tcp: Avoid socket skips and repeats during
iteration"), remove unnecessary bucket bounds checks in
bpf_iter_tcp_resume. In either case, if st->bucket is outside the
current table's range then bpf_iter_tcp_resume_* calls *_get_first
which immediately returns NULL anyway and the logic will fall through.
(Martin)
* Add a check at the top of bpf_iter_tcp_resume_listening and
bpf_iter_tcp_resume_established to see if we're done with the current
bucket and advance it immediately instead of wasting time finding the
first matching socket in that bucket with
(listening|established)_get_first. In v1, we originally discussed
adding logic to advance the bucket in bpf_iter_tcp_seq_next and
bpf_iter_tcp_seq_stop, but after trying this the logic seemed harder
to track. Overall, keeping everything inside bpf_iter_tcp_resume_*
seemed a bit clearer. (Martin)
* Instead of using a timeout in the last patch ("selftests/bpf: Add
tests for bucket resume logic in established sockets") to wait for
sockets to leave the ehash table after calling close(), use
bpf_sock_destroy to deterministically destroy and remove them. This
introduces one more patch ("selftests/bpf: Create iter_tcp_destroy
test program") to create the iterator program that destroys a selected
socket. Drive this through a destroy() function in the last patch
which, just like close(), accepts a socket file descriptor. (Martin)
* Introduce one more patch ("selftests/bpf: Allow for iteration over
multiple states") to fix a latent bug in iter_tcp_soreuse where the
sk->sk_state != TCP_LISTEN check was ignored. Add the "ss" variable to
allow test code to configure which socket states to allow.
[1]: https://lore.kernel.org/bpf/20250502161528.264630-1-jordan@jrife.io/
====================
Link: https://patch.msgid.link/20250714180919.127192-1-jordan@jrife.io
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
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Linux kernel
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