Re: [PATCH v2 bpf-next 1/5] bpf: add in-kernel split BTF support

[Song Liu]

> On Nov 6, 2020, at 3:02 PM, Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
> 
> Adjust in-kernel BTF implementation to support a split BTF mode of operation.
> Changes are mostly mirroring libbpf split BTF changes, with the exception of
> start_id being 0 for in-kernel implementation due to simpler read-only mode.
> 
> Otherwise, for split BTF logic, most of the logic of jumping to base BTF,
> where necessary, is encapsulated in few helper functions. Type numbering and
> string offset in a split BTF are logically continuing where base BTF ends, so
> most of the high-level logic is kept without changes.
> 
> Type verification and size resolution is only doing an added resolution of new
> split BTF types and relies on already cached size and type resolution results
> in the base BTF.
> 
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

[...]

> 
> @@ -600,8 +618,15 @@ static const struct btf_kind_operations *btf_type_ops(const struct btf_type *t)
> 
> static bool btf_name_offset_valid(const struct btf *btf, u32 offset)
> {
> -	return BTF_STR_OFFSET_VALID(offset) &&
> -		offset < btf->hdr.str_len;
> +	if (!BTF_STR_OFFSET_VALID(offset))
> +		return false;
> +again:
> +	if (offset < btf->start_str_off) {
> +		btf = btf->base_btf;
> +		goto again;

Can we do a while loop instead of "goto again;"?

> +	}
> +	offset -= btf->start_str_off;
> +	return offset < btf->hdr.str_len;
> }
> 
> static bool __btf_name_char_ok(char c, bool first, bool dot_ok)
> @@ -615,10 +640,25 @@ static bool __btf_name_char_ok(char c, bool first, bool dot_ok)
> 	return true;
> }
> 
> +static const char *btf_str_by_offset(const struct btf *btf, u32 offset)
> +{
> +again:
> +	if (offset < btf->start_str_off) {
> +		btf = btf->base_btf;
> +		goto again;
> +	}

Maybe add a btf_find_base_btf(btf, offset) helper for this logic?

> +
> +	offset -= btf->start_str_off;
> +	if (offset < btf->hdr.str_len)
> +		return &btf->strings[offset];
> +
> +	return NULL;
> +}
> +

[...]

> }
> 
> const char *btf_name_by_offset(const struct btf *btf, u32 offset)
> {
> -	if (offset < btf->hdr.str_len)
> -		return &btf->strings[offset];
> -
> -	return NULL;
> +	return btf_str_by_offset(btf, offset);
> }

IIUC, btf_str_by_offset() and btf_name_by_offset() are identical. Can we
just keep btf_name_by_offset()?

> 
> const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id)
> {
> -	if (type_id > btf->nr_types)
> -		return NULL;
> +again:
> +	if (type_id < btf->start_id) {
> +		btf = btf->base_btf;
> +		goto again;
> +	}

ditto, goto again..

[...]