perf evlist: Enforce ring buffer reading

	struct perf_mmap *md = &evlist->mmap[idx];

	u64 head;

	u64 old = md->prev;

	int diff;

	unsigned char *data = md->base + page_size;

	union perf_event *event = NULL;

		return NULL;

	head = perf_mmap__read_head(md);

	diff = head - old;

	if (evlist->overwrite) {

		/*

		 * If we're further behind than half the buffer, there's a chance

		 *

		 * In either case, truncate and restart at head.

		 */

		int diff = head - old;

		if (diff > md->mask / 2 || diff < 0) {

			fprintf(stderr, "WARNING: failed to keep up with mmap data.\n");

			 * head points to a known good entry, start there.

			 */

			old = head;

			diff = 0;

		}

	}

	if (old != head) {

	if (diff >= (int)sizeof(event->header)) {

		size_t size;

		event = (union perf_event *)&data[old & md->mask];

		size = event->header.size;

		if (size < sizeof(event->header) || diff < (int)size) {

			event = NULL;

			goto broken_event;

		}

		/*

		 * Event straddles the mmap boundary -- header should always

		 * be inside due to u64 alignment of output.

		old += size;

	}

broken_event:

	md->prev = old;

	return event;