Subject: Re: [PATCH bpf-next v5 03/11] bpf: Add write access to tcp_sock and sock fields

From: Eric Dumazet <[email-masked]>
Date: Tue, 09 Jan 2018 15:21:48 -0800

On Tue, 2018-01-09 at 13:06 -0800, Lawrence Brakmo wrote:
> This patch adds a macro, SOCK_OPS_SET_FIELD, for writing to
> struct tcp_sock or struct sock fields. This required adding a new
> field "temp" to struct bpf_sock_ops_kern for temporary storage that
> is used by sock_ops_convert_ctx_access. It is used to store and recover
> the contents of a register, so the register can be used to store the
> address of the sk. Since we cannot overwrite the dst_reg because it
> contains the pointer to ctx, nor the src_reg since it contains the value
> we want to store, we need an extra register to contain the address
> of the sk.
> 
> Also adds the macro SOCK_OPS_GET_OR_SET_FIELD that calls one of the
> GET or SET macros depending on the value of the TYPE field.
> 
> Signed-off-by: Lawrence Brakmo <[email-masked]>
> ---
>  include/linux/filter.h |  3 +++
>  include/net/tcp.h      |  2 +-
>  net/core/filter.c      | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 425056c..139a9a3 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -1007,6 +1007,9 @@ struct bpf_sock_ops_kern {
>  		u32 replylong[4];
>  	};
>  	u32	is_fullsock;
> +	u64	temp;			/* Used by sock_ops_convert_ctx_access
> +					 * as temporary storaage of a register

typo on storage
	
> +					 */
>  };
>  
>  #endif /* __LINUX_FILTER_H__ */
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 6939e69..108d16a 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -2010,7 +2010,7 @@ static inline int tcp_call_bpf(struct sock *sk, int op)
>  	struct bpf_sock_ops_kern sock_ops;
>  	int ret;
>  
> -	memset(&sock_ops, 0, sizeof(sock_ops));
> +	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
> 

This lacks proper comment, stating that every field added after .temp
in struct bpf_sock_ops_kern is considered as scratch and could contain
garbage.