Subject: Re: [PATCH bpf-next 04/13] bpf: Support bitfield read access in btf_struct_access

From: Yonghong Song <[email-masked]>
Date: Mon, 16 Dec 2019 22:05:47 +0000

On 12/13/19 4:47 PM, Martin KaFai Lau wrote:
> This patch allows bitfield access as a scalar.  It currently limits
> the access to sizeof(u64) and upto the end of the struct.  It is needed
> in a later bpf-tcp-cc example that reads bitfield from
> inet_connection_sock and tcp_sock.
> 
> Signed-off-by: Martin KaFai Lau <[email-masked]>
> ---
>   kernel/bpf/btf.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 6e652643849b..011194831499 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3744,10 +3744,6 @@ int btf_struct_access(struct bpf_verifier_log *log,
>   	}
>   
>   	for_each_member(i, t, member) {
> -		if (btf_member_bitfield_size(t, member))
> -			/* bitfields are not supported yet */
> -			continue;
> -
>   		/* offset of the field in bytes */
>   		moff = btf_member_bit_offset(t, member) / 8;
>   		if (off + size <= moff)
> @@ -3757,6 +3753,15 @@ int btf_struct_access(struct bpf_verifier_log *log,
>   		if (off < moff)
>   			continue;
>   
> +		if (btf_member_bitfield_size(t, member)) {
> +			if (off == moff &&
> +			    !(btf_member_bit_offset(t, member) % 8) &&

This check '!(btf_member_bit_offset(t, member) % 8)' is not needed.

> +			    size <= sizeof(u64) &&

This one is not needed since verifier gets 'size' from load/store 
instructions which is guaranteed to be <= sizeof(u64).

> +			    off + size <= t->size)
> +				return SCALAR_VALUE;
> +			continue;
> +		}
> +
>   		/* type of the field */
>   		mtype = btf_type_by_id(btf_vmlinux, member->type);
>   		mname = __btf_name_by_offset(btf_vmlinux, member->name_off);
>