Before this patch, the producer position could overflow `unsigned long`, in which case libbpf would forever stop processing new writes to the ringbuf. This patch addresses that bug by avoiding ordered comparison between the consumer and producer position. If the consumer position is greater than the producer position, the assumption is that the producer has overflowed. A more defensive check could be to ensure that the delta is within the allowed range, but such defensive checks are neither present in the kernel side code nor in libbpf. The overflow that this patch handles can occur while the producer and consumer follow a correct protocol. A selftest was written to demonstrate the bug, and indeed this patch allows the test to continue to make progress past the overflow. However, the author was unable to create a testing environment on a 32-bit machine, and the test requires substantial memory and over 4 hours to hit the overflow point on a 64-bit machine. Thus, the test is not included in this patch because of the impracticality of running it. Additionally, this patch adds commentary around a separate point to note that the modular arithmetic is valid in the face of overflows, as that fact may not be obvious to future readers. v1->v2: - Fixed comment grammar. - Properly formatted subject line. Reference: [v1]: https://lore.kernel.org/bpf/20230724132404.1280848-1-awerner32@xxxxxxxxx/T/#u Signed-off-by: Andrew Werner <awerner32@xxxxxxxxx> --- tools/lib/bpf/ringbuf.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c index 02199364db13..2055f3099843 100644 --- a/tools/lib/bpf/ringbuf.c +++ b/tools/lib/bpf/ringbuf.c @@ -237,7 +237,11 @@ static int64_t ringbuf_process_ring(struct ring *r) do { got_new_data = false; prod_pos = smp_load_acquire(r->producer_pos); - while (cons_pos < prod_pos) { + + /* Avoid signed comparisons between the positions; the producer + * position can overflow before the consumer position. + */ + while (cons_pos != prod_pos) { len_ptr = r->data + (cons_pos & r->mask); len = smp_load_acquire(len_ptr); @@ -498,6 +502,11 @@ void *user_ring_buffer__reserve(struct user_ring_buffer *rb, __u32 size) prod_pos = smp_load_acquire(rb->producer_pos); max_size = rb->mask + 1; + + /* Note that this formulation is valid in the face of overflow of + * prod_pos so long as the delta between prod_pos and cons_pos is + * no greater than max_size. + */ avail_size = max_size - (prod_pos - cons_pos); /* Round up total size to a multiple of 8. */ total_size = (size + BPF_RINGBUF_HDR_SZ + 7) / 8 * 8; -- 2.39.2
