Re: [PATCH bpf-next 4/7] bpf: btf: Remove unused bits from uapi/linux/btf.h

[Yonghong Song <yhs@fb.com>]

On 5/18/18 5:16 PM, Martin KaFai Lau wrote:
> This patch does the followings:
> 1. Limit BTF_MAX_TYPES and BTF_MAX_NAME_OFFSET to 64k.  We can
>     raise it later.
> 
> 2. Remove the BTF_TYPE_PARENT and BTF_STR_TBL_ELF_ID.  They are
>     currently encoded at the highest bit of a u32.
>     It is because the current use case does not require supporting
>     parent type (i.e type_id referring to a type in another BTF file).
>     It also does not support referring to a string in ELF.
> 
>     The BTF_TYPE_PARENT and BTF_STR_TBL_ELF_ID checks are replaced
>     by BTF_TYPE_ID_CHECK and BTF_STR_OFFSET_CHECK which are
>     defined in btf.c instead of uapi/linux/btf.h.
> 
> 3. Limit the BTF_INFO_KIND from 5 bits to 4 bits which is enough.
>     There is unused bits headroom if we ever needed it later.
> 
> 4. The root bit in BTF_INFO is also removed because it is not
>     used in the current use case.
> 
> The above can be added back later because the verifier
> ensures the unused bits are zeros.
> 
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>

Acked-by: Yonghong Song <yhs@fb.com>