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Message-ID: <20200514165045.sk6zjlhsfzxbo6mb@bsd-mbp>
Date: Thu, 14 May 2020 09:50:45 -0700
From: Jonathan Lemon <bsd@...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:
> +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
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