netdev - Re: [PATCH bpf-next 1/6] bpf: implement BPF ring buffer and verifier support for it

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Date:   Thu, 14 May 2020 12:06:49 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, andrii.nakryiko@...il.com,
        kernel-team@...com, "Paul E . McKenney" <paulmck@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH bpf-next 1/6] bpf: implement BPF ring buffer and verifier
 support for it

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?