[PATCH v2 bpf-next] libbpf: Fix signed overflow in ringbuf_process_ring

[Brendan Jackman <jackmanb@google.com>]

One of our benchmarks running in (Google-internal) CI pushes data
through the ringbuf faster htan than userspace is able to consume
it. In this case it seems we're actually able to get >INT_MAX entries
in a single ringbuf_buffer__consume call. ASAN detected that cnt
overflows in this case.

Fix by using 64-bit counter internally and then capping the result to
INT_MAX before converting to the int return type.

Fixes: bf99c936f947 (libbpf: Add BPF ring buffer support)
Signed-off-by: Brendan Jackman <jackmanb@google.com>
---

diff v1->v2: Now we don't break the loop at INT_MAX, we just cap the reported
entry count.

Note: I feel a bit guilty about the fact that this makes the reader
think about implicit conversions. Nobody likes thinking about that.

But explicit casts don't really help with clarity:

  return (int)min(cnt, (int64_t)INT_MAX); // ugh

shrug..

 tools/lib/bpf/ringbuf.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
index e7a8d847161f..2e114c2d0047 100644
--- a/tools/lib/bpf/ringbuf.c
+++ b/tools/lib/bpf/ringbuf.c
@@ -204,7 +204,9 @@ static inline int roundup_len(__u32 len)

 static int ringbuf_process_ring(struct ring* r)
 {
-	int *len_ptr, len, err, cnt = 0;
+	int *len_ptr, len, err;
+	/* 64-bit to avoid overflow in case of extreme application behavior */
+	int64_t cnt = 0;
 	unsigned long cons_pos, prod_pos;
 	bool got_new_data;
 	void *sample;
@@ -240,7 +242,7 @@ static int ringbuf_process_ring(struct ring* r)
 		}
 	} while (got_new_data);
 done:
-	return cnt;
+	return min(cnt, INT_MAX);
 }

 /* Consume available ring buffer(s) data without event polling.
@@ -263,8 +265,8 @@ int ring_buffer__consume(struct ring_buffer *rb)
 }

 /* Poll for available data and consume records, if any are available.
- * Returns number of records consumed, or negative number, if any of the
- * registered callbacks returned error.
+ * Returns number of records consumed (or INT_MAX, whichever is less), or
+ * negative number, if any of the registered callbacks returned error.
  */
 int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
 {
--
2.31.1.498.g6c1eba8ee3d-goog