On Wed, May 13, 2020 at 12:25:27PM -0700, Andrii Nakryiko wrote:

+static struct bpf_ringbuf *bpf_ringbuf_restore_from_rec(void *meta_ptr)
+{
+	unsigned long addr = (unsigned long)meta_ptr;
+	unsigned long off = *(u32 *)(meta_ptr + 4) << PAGE_SHIFT;
+
+	return (void*)((addr & PAGE_MASK) - off);
+}
+
+static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
+{
+	unsigned long cons_pos, prod_pos, new_prod_pos, flags;
+	u32 len, pg_off;
+	void *meta_ptr;
+
+	if (unlikely(size > UINT_MAX))
+		return NULL;

Size should be 30 bits, not UINT_MAX, since 2 bits are reserved.

+
+	len = round_up(size + RINGBUF_META_SZ, 8);
+	cons_pos = READ_ONCE(rb->consumer_pos);
+
+	if (in_nmi()) {
+		if (!spin_trylock_irqsave(&rb->spinlock, flags))
+			return NULL;
+	} else {
+		spin_lock_irqsave(&rb->spinlock, flags);
+	}
+
+	prod_pos = rb->producer_pos;
+	new_prod_pos = prod_pos + len;
+
+	/* check for out of ringbuf space by ensuring producer position
+	 * doesn't advance more than (ringbuf_size - 1) ahead
+	 */
+	if (new_prod_pos - cons_pos > rb->mask) {
+		spin_unlock_irqrestore(&rb->spinlock, flags);
+		return NULL;
+	}
+
+	meta_ptr = rb->data + (prod_pos & rb->mask);
+	pg_off = bpf_ringbuf_rec_pg_off(rb, meta_ptr);
+
+	WRITE_ONCE(*(u32 *)meta_ptr, RINGBUF_BUSY_BIT | size);
+	WRITE_ONCE(*(u32 *)(meta_ptr + 4), pg_off);

Or define a 64bit word in the structure and use:

        WRITE_ONCE(*(u64 *)meta_ptr, rec.header);

+
+	/* ensure length prefix is written before updating producer positions */
+	smp_wmb();
+	WRITE_ONCE(rb->producer_pos, new_prod_pos);
+
+	spin_unlock_irqrestore(&rb->spinlock, flags);
+
+	return meta_ptr + RINGBUF_META_SZ;
+}
+
+BPF_CALL_3(bpf_ringbuf_reserve, struct bpf_map *, map, u64, size, u64, flags)
+{
+	struct bpf_ringbuf_map *rb_map;
+
+	if (unlikely(flags))
+		return -EINVAL;
+
+	rb_map = container_of(map, struct bpf_ringbuf_map, map);
+	return (unsigned long)__bpf_ringbuf_reserve(rb_map->rb, size);
+}
+

--
Jonathan