bpf: Fix available-data accounting on 32-bit wrap in overwrite mode

In overwrite mode ringbuf_avail_data_sz() picks the newer of the consumer
and overwrite positions before measuring how much data is available:

	return prod_pos - max(cons_pos, over_pos);

max() is an ordering comparison, and consumer_pos, producer_pos and
overwrite_pos are unsigned long, i.e. 32-bit on 32-bit architectures,
where Documentation/bpf/ringbuf.rst allows them to wrap. Once one of the
two positions has wrapped and the other has not, max() returns the older
one: the result is then a modular difference close to 2^32, so the
function reports far more available data than the ring can hold. Pollers
using BPF_RB_AVAIL_DATA get a bogus figure, and epoll consumers can be
woken with nothing to read.

Compare distances rather than positions. prod_pos - X is the amount of
data produced since X for either position, wrap or no wrap, so the newer
position is simply the one with the smaller distance, which is also the
value the function wants to return.

64-bit hosts are unaffected in practice: their counters would need
16 EiB to wrap. Found by review of the same class of bug fixed in
"bpf: Fix pending_pos walk on 32-bit ring position wrap".

Signed-off-by: Israel Téllez García <i.tellez@btesa.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260814124843.22041-3-i.tellez@btesa.com

kernel/bpf/ringbuf.c

@@ -321,7 +321,7 @@ static unsigned long ringbuf_avail_data_sz(struct bpf_ringbuf *rb)
 	if (unlikely(rb->overwrite_mode)) {
 		over_pos = smp_load_acquire(&rb->overwrite_pos);
 		prod_pos = smp_load_acquire(&rb->producer_pos);
-		return prod_pos - max(cons_pos, over_pos);
+		return min(prod_pos - cons_pos, prod_pos - over_pos);
 	} else {
 		prod_pos = smp_load_acquire(&rb->producer_pos);
 		return prod_pos - cons_pos;