Re: [PATCH v2 bpf 4/4] libbpf: Fix ring buffer consumer loop on 32-bit position wrap

> libbpf: Fix ring buffer consumer loop on 32-bit position wrap
>
> ringbuf_process_ring() walks the records between the consumer and the
> producer with an ordering comparison:
>
> 	while (cons_pos < prod_pos) {
>
> cons_pos and prod_pos mirror the kernel's ring positions and are
> unsigned long here too, so on 32-bit they wrap at 2^32 bytes of traffic.
> When producer_pos has wrapped and consumer_pos has not, prod_pos is the
> smaller of the two, the loop body never runs and no record is consumed.
> Since consumer_pos only advances inside that loop, it never wraps either
> and the consumer stops delivering samples for good, with no error
> returned to the caller: ring_buffer__poll() keeps reporting zero
> records while the kernel side fills up and starts dropping.
>
> Compare the distance instead. The consumer never runs ahead of the
> producer, so prod_pos - cons_pos is the amount of unconsumed data and
> stays correct across the wrap.
>
> 64-bit hosts are unaffected in practice: the counters would need 16 EiB
> to wrap. This is the userspace counterpart of the kernel-side walk fixed
> in "bpf: Fix pending_pos walk on 32-bit ring position wrap"; a 32-bit
> consumer hits whichever of the two comes first.
>
> Signed-off-by: Israel Téllez García <[email protected]>

This looks like a bug fix for ring buffer support. Should this include:

  Fixes: bf99c936f947 ("libbpf: Add BPF ring buffer support")

The buggy comparison 'while (cons_pos < prod_pos)' was introduced when
ring buffer support was first added to libbpf. On 32-bit systems, this
comparison fails when producer_pos wraps, causing the consumer to stop
delivering samples permanently.


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