linux-kernel - Re: [PATCH v7 bpf-next 8/9] bpf: introduce bpf_jit_binary_pack

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Date:   Tue, 1 Feb 2022 01:21:05 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Song Liu <song@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     ast@...nel.org, andrii@...nel.org, kernel-team@...com,
        peterz@...radead.org, x86@...nel.org, iii@...ux.ibm.com,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH v7 bpf-next 8/9] bpf: introduce
 bpf_jit_binary_pack_[alloc|finalize|free]

On 1/29/22 12:45 AM, Song Liu wrote:
[...]
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 25e34caa9a95..ff0c51ef1cb7 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1031,6 +1031,109 @@ void bpf_jit_binary_free(struct bpf_binary_header *hdr)
>   	bpf_jit_uncharge_modmem(size);
>   }
>   
> +/* Allocate jit binary from bpf_prog_pack allocator.
> + * Since the allocated meory is RO+X, the JIT engine cannot write directly

nit: meory

> + * to the memory. To solve this problem, a RW buffer is also allocated at
> + * as the same time. The JIT engine should calculate offsets based on the
> + * RO memory address, but write JITed program to the RW buffer. Once the
> + * JIT engine finishes, it calls bpf_jit_binary_pack_finalize, which copies
> + * the JITed program to the RO memory.
> + */
> +struct bpf_binary_header *
> +bpf_jit_binary_pack_alloc(unsigned int proglen, u8 **image_ptr,
> +			  unsigned int alignment,
> +			  struct bpf_binary_header **rw_header,
> +			  u8 **rw_image,
> +			  bpf_jit_fill_hole_t bpf_fill_ill_insns)
> +{
> +	struct bpf_binary_header *ro_header;
> +	u32 size, hole, start;
> +
> +	WARN_ON_ONCE(!is_power_of_2(alignment) ||
> +		     alignment > BPF_IMAGE_ALIGNMENT);
> +
> +	/* add 16 bytes for a random section of illegal instructions */
> +	size = round_up(proglen + sizeof(*ro_header) + 16, BPF_PROG_CHUNK_SIZE);
> +
> +	if (bpf_jit_charge_modmem(size))
> +		return NULL;
> +	ro_header = bpf_prog_pack_alloc(size);
> +	if (!ro_header) {
> +		bpf_jit_uncharge_modmem(size);
> +		return NULL;
> +	}
> +
> +	*rw_header = kvmalloc(size, GFP_KERNEL);
> +	if (!*rw_header) {
> +		bpf_prog_pack_free(ro_header);
> +		bpf_jit_uncharge_modmem(size);
> +		return NULL;
> +	}
> +
> +	/* Fill space with illegal/arch-dep instructions. */
> +	bpf_fill_ill_insns(*rw_header, size);
> +	(*rw_header)->size = size;
> +
> +	hole = min_t(unsigned int, size - (proglen + sizeof(*ro_header)),
> +		     BPF_PROG_CHUNK_SIZE - sizeof(*ro_header));
> +	start = (get_random_int() % hole) & ~(alignment - 1);
> +
> +	*image_ptr = &ro_header->image[start];
> +	*rw_image = &(*rw_header)->image[start];
> +
> +	return ro_header;
> +}
> +
> +/* Copy JITed text from rw_header to its final location, the ro_header. */
> +int bpf_jit_binary_pack_finalize(struct bpf_prog *prog,
> +				 struct bpf_binary_header *ro_header,
> +				 struct bpf_binary_header *rw_header)
> +{
> +	void *ptr;
> +
> +	ptr = bpf_arch_text_copy(ro_header, rw_header, rw_header->size);

Does this need to be wrapped with a text_mutex lock/unlock pair given
text_poke_copy() internally relies on __text_poke() ?

> +	kvfree(rw_header);
> +
> +	if (IS_ERR(ptr)) {
> +		bpf_prog_pack_free(ro_header);
> +		return PTR_ERR(ptr);
> +	}
> +	prog->aux->use_bpf_prog_pack = true;
> +	return 0;
> +}
> +
[...]