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.
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..6271757bc3d2 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 in the face of overflow of prod_pos
+ * so long as the delta between prod_pos and cons_pos remains 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
