RE: [PATCH bpf-next 1/2] bpf: Add bits iterator

[John Fastabend <john.fastabend@gmail.com>]

Yafang Shao wrote:
> Add three new kfuncs for the bits iterator:
> - bpf_iter_bits_new
>   Initialize a new bits iterator for a given memory area. Due to the
>   limitation of bpf memalloc, the max number of bits that can be iterated
>   over is limited to (4096 * 8).
> - bpf_iter_bits_next
>   Get the next bit in a bpf_iter_bits
> - bpf_iter_bits_destroy
>   Destroy a bpf_iter_bits
> 
> The bits iterator facilitates the iteration of the bits of a memory area,
> such as cpumask. It can be used in any context and on any address.

Just curious as I see more and a more kfuncs. Did you try to implement
this with existing BPF? The main trick looks to be to get an implementation
of FIND_NEXT_BIT? Without trying seems doable with one of the bpf loop
iterators?

Also this requires a bpf_iter_bits_new across every iteration of the
BPF program or anytime we need to pick up the changes. Any reason
we can't just read the memory directly?

Thanks,
John

> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  kernel/bpf/helpers.c | 100 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 100 insertions(+)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 93edf730d288..052f63891834 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2542,6 +2542,103 @@ __bpf_kfunc void bpf_throw(u64 cookie)
>  	WARN(1, "A call to BPF exception callback should never return\n");
>  }
>  
> +struct bpf_iter_bits {
> +	__u64 __opaque[2];
> +} __aligned(8);
> +
> +struct bpf_iter_bits_kern {
> +	unsigned long *bits;
> +	u32 nr_bits;
> +	int bit;
> +} __aligned(8);
> +
> +/**
> + * bpf_iter_bits_new() - Initialize a new bits iterator for a given memory area
> + * @it: The new bpf_iter_bits to be created
> + * @unsafe_ptr__ign: A ponter pointing to a memory area to be iterated over
> + * @nr_bits: The number of bits to be iterated over. Due to the limitation of
> + * memalloc, it can't greater than (4096 * 8).
> + *
> + * This function initializes a new bpf_iter_bits structure for iterating over
> + * a memory area which is specified by the @unsafe_ptr__ign and @nr_bits. It
> + * copy the data of the memory area to the newly created bpf_iter_bits @it for
> + * subsequent iteration operations.
> + *
> + * On success, 0 is returned. On failure, ERR is returned.
> + */
> +__bpf_kfunc int
> +bpf_iter_bits_new(struct bpf_iter_bits *it, const void *unsafe_ptr__ign, u32 nr_bits)
> +{
> +	struct bpf_iter_bits_kern *kit = (void *)it;
> +	u32 size = BITS_TO_BYTES(nr_bits);
> +	int err;
> +
> +	BUILD_BUG_ON(sizeof(struct bpf_iter_bits_kern) != sizeof(struct bpf_iter_bits));
> +	BUILD_BUG_ON(__alignof__(struct bpf_iter_bits_kern) !=
> +		     __alignof__(struct bpf_iter_bits));
> +
> +	if (!unsafe_ptr__ign || !nr_bits) {
> +		kit->bits = NULL;
> +		return -EINVAL;
> +	}
> +
> +	kit->bits = bpf_mem_alloc(&bpf_global_ma, size);
> +	if (!kit->bits)
> +		return -ENOMEM;
> +
> +	err = bpf_probe_read_kernel_common(kit->bits, size, unsafe_ptr__ign);

Specifically, this why can't we iterate over unsafe_ptr__ign?

> +	if (err) {
> +		bpf_mem_free(&bpf_global_ma, kit->bits);
> +		kit->bits = NULL;
> +		return err;
> +	}
> +
> +	kit->nr_bits = nr_bits;
> +	kit->bit = -1;
> +	return 0;
> +}
> +
> +/**
> + * bpf_iter_bits_next() - Get the next bit in a bpf_iter_bits
> + * @it: The bpf_iter_bits to be checked
> + *
> + * This function returns a pointer to a number representing the value of the
> + * next bit in the bits.
> + *
> + * If there are no further bit available, it returns NULL.
> + */
> +__bpf_kfunc int *bpf_iter_bits_next(struct bpf_iter_bits *it)
> +{
> +	struct bpf_iter_bits_kern *kit = (void *)it;
> +	const unsigned long *bits = kit->bits;
> +	int bit;
> +
> +	if (!bits)
> +		return NULL;
> +
> +	bit = find_next_bit(bits, kit->nr_bits, kit->bit + 1);

Seems like this should be ok over unsafe memory as long as find_next_bit
is bounded?

> +	if (bit >= kit->nr_bits)
> +		return NULL;
> +
> +	kit->bit = bit;
> +	return &kit->bit;
> +}


Thanks for working on this looks useful to me.