Re: [PATCH v1 bpf-next 02/10] bpf: tcp: Make sure iter->batch always contains a full bucket snapshot

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Jordan Rife <jordan@jrife.io>
Date: Tue, 20 May 2025 07:50:49 -0700
> Require that iter->batch always contains a full bucket snapshot. This
> invariant is important to avoid skipping or repeating sockets during
> iteration when combined with the next few patches. Before, there were
> two cases where a call to bpf_iter_tcp_batch may only capture part of a
> bucket:
> 
> 1. When bpf_iter_tcp_realloc_batch() returns -ENOMEM.
> 2. When more sockets are added to the bucket while calling
>    bpf_iter_tcp_realloc_batch(), making the updated batch size
>    insufficient.
> 
> In cases where the batch size only covers part of a bucket, it is
> possible to forget which sockets were already visited, especially if we
> have to process a bucket in more than two batches. This forces us to
> choose between repeating or skipping sockets, so don't allow this:
> 
> 1. Stop iteration and propagate -ENOMEM up to userspace if reallocation
>    fails instead of continuing with a partial batch.
> 2. Try bpf_iter_tcp_realloc_batch() with GFP_USER just as before, but if
>    we still aren't able to capture the full bucket, call
>    bpf_iter_tcp_realloc_batch() again while holding the bucket lock to
>    guarantee the bucket does not change. On the second attempt use
>    GFP_NOWAIT since we hold onto the spin lock.
> 
> I did some manual testing to exercise the code paths where GFP_NOWAIT is
> used and where ERR_PTR(err) is returned. I used the realloc test cases
> included later in this series to trigger a scenario where a realloc
> happens inside bpf_iter_tcp_batch and made a small code tweak to force
> the first realloc attempt to allocate a too-small batch, thus requiring
> another attempt with GFP_NOWAIT. Some printks showed both reallocs with
> the tests passing:
> 
> May 09 18:18:55 crow kernel: resize batch TCP_SEQ_STATE_LISTENING
> May 09 18:18:55 crow kernel: again GFP_USER
> May 09 18:18:55 crow kernel: resize batch TCP_SEQ_STATE_LISTENING
> May 09 18:18:55 crow kernel: again GFP_NOWAIT
> May 09 18:18:57 crow kernel: resize batch TCP_SEQ_STATE_ESTABLISHED
> May 09 18:18:57 crow kernel: again GFP_USER
> May 09 18:18:57 crow kernel: resize batch TCP_SEQ_STATE_ESTABLISHED
> May 09 18:18:57 crow kernel: again GFP_NOWAIT
> 
> With this setup, I also forced each of the bpf_iter_tcp_realloc_batch
> calls to return -ENOMEM to ensure that iteration ends and that the
> read() in userspace fails.
> 
> Signed-off-by: Jordan Rife <jordan@jrife.io>

Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>