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

+
+/* Given pointer to ring buffer record metadata, restore pointer to struct
+ * bpf_ringbuf itself by using page offset stored at offset 4
+ */
+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;

Looking at the further code it seems this one should be READ_ONCE, but...

+
+	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;
+
+	len = round_up(size + RINGBUF_META_SZ, 8);

it may overflow despite the check above.

+	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);

it doens't match to few other places where normal read is done.
But why WRITE_ONCE here?
How does it race with anything?
producer_pos is updated later.

+
+	/* ensure length prefix is written before updating producer positions */
+	smp_wmb();

this barrier is enough to make sure meta_ptr and meta_ptr+4 init
is visible before producer_pos is updated below.

+	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);
+}
+
+const struct bpf_func_proto bpf_ringbuf_reserve_proto = {
+	.func		= bpf_ringbuf_reserve,
+	.ret_type	= RET_PTR_TO_ALLOC_MEM_OR_NULL,
+	.arg1_type	= ARG_CONST_MAP_PTR,
+	.arg2_type	= ARG_CONST_ALLOC_SIZE_OR_ZERO,
+	.arg3_type	= ARG_ANYTHING,
+};
+
+static void bpf_ringbuf_commit(void *sample, bool discard)
+{
+	unsigned long rec_pos, cons_pos;
+	u32 new_meta, old_meta;
+	void *meta_ptr;
+	struct bpf_ringbuf *rb;
+
+	meta_ptr = sample - RINGBUF_META_SZ;
+	rb = bpf_ringbuf_restore_from_rec(meta_ptr);
+	old_meta = *(u32 *)meta_ptr;

I think this one will race with user space and should be READ_ONCE.

+	new_meta = old_meta ^ RINGBUF_BUSY_BIT;
+	if (discard)
+		new_meta |= RINGBUF_DISCARD_BIT;
+
+	/* update metadata header with correct final size prefix */
+	xchg((u32 *)meta_ptr, new_meta);
+
+	/* if consumer caught up and is waiting for our record, notify about
+	 * new data availability
+	 */
+	rec_pos = (void *)meta_ptr - (void *)rb->data;
+	cons_pos = smp_load_acquire(&rb->consumer_pos) & rb->mask;

hmm. Earlier WRITE_ONCE(rb->producer_pos) is used, but here it's load_acquire.
Please be consistent with pairing.

+	if (cons_pos == rec_pos)
+		wake_up_all(&rb->waitq);

Is it legal to do from preempt_disabled region?