netdev - RE: [PATCH bpf-next 2/9] libbpf: remove assumption of single contiguous memory for BTF data

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Date:   Thu, 24 Sep 2020 08:21:27 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...com, daniel@...earbox.net
Cc:     andrii.nakryiko@...il.com, kernel-team@...com,
        Andrii Nakryiko <andriin@...com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: RE: [PATCH bpf-next 2/9] libbpf: remove assumption of single
 contiguous memory for BTF data

Andrii Nakryiko wrote:
> Refactor internals of struct btf to remove assumptions that BTF header, type
> data, and string data are layed out contiguously in a memory in a single
> memory allocation. Now we have three separate pointers pointing to the start
> of each respective are: header, types, strings. In the next patches, these
> pointers will be re-assigned to point to independently allocated memory areas,
> if BTF needs to be modified.
> 
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---

[...]

> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -27,18 +27,37 @@
>  static struct btf_type btf_void;
>  
>  struct btf {
> -	union {
> -		struct btf_header *hdr;
> -		void *data;
> -	};
> +	void *raw_data;
> +	__u32 raw_size;
> +
> +	/*
> +	 * When BTF is loaded from ELF or raw memory it is stored
> +	 * in contiguous memory block, pointed to by raw_data pointer, and
> +	 * hdr, types_data, and strs_data point inside that memory region to
> +	 * respective parts of BTF representation:

I find the above comment a bit confusing. The picture though is great. How
about something like,

  When BTF is loaded from an ELF or raw memory it is stored
  in a continguous memory block. The hdr, type_data, and strs_data
  point inside that memory region to their respective parts of BTF
  representation

> +	 *
> +	 * +--------------------------------+
> +	 * |  Header  |  Types  |  Strings  |
> +	 * +--------------------------------+
> +	 * ^          ^         ^
> +	 * |          |         |
> +	 * hdr        |         |
> +	 * types_data-+         |
> +	 * strs_data------------+
> +	 */
> +	struct btf_header *hdr;
> +	void *types_data;
> +	void *strs_data;
> +
> +	/* type ID to `struct btf_type *` lookup index */
>  	__u32 *type_offs;
>  	__u32 type_offs_cap;
> -	const char *strings;
> -	void *nohdr_data;
> -	void *types_data;
>  	__u32 nr_types;
> -	__u32 data_size;
> +
> +	/* BTF object FD, if loaded into kernel */
>  	int fd;
> +
> +	/* Pointer size (in bytes) for a target architecture of this BTF */
>  	int ptr_sz;
>  };
>  

Thanks,
John