linux-kernel - Re: [PATCH bpf-next] bpf: getsockopt hook to get optval without checking kernel retval

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Date:   Wed, 31 May 2023 22:37:52 -0700
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     Feng zhou <zhoufeng.zf@...edance.com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        yangzhenze@...edance.com, wangdongdong.6@...edance.com,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        song@...nel.org, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org
Subject: Re: [PATCH bpf-next] bpf: getsockopt hook to get optval without
 checking kernel retval

On 5/31/23 7:49 PM, Feng zhou wrote:
> From: Feng Zhou <zhoufeng.zf@...edance.com>
> 
> Remove the judgment on retval and pass bpf ctx by default. The
> advantage of this is that it is more flexible. Bpf getsockopt can
> support the new optname without using the module to call the
> nf_register_sockopt to register.
> 
> Signed-off-by: Feng Zhou <zhoufeng.zf@...edance.com>
> ---
>   kernel/bpf/cgroup.c | 35 +++++++++++++----------------------
>   1 file changed, 13 insertions(+), 22 deletions(-)
> 
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 5b2741aa0d9b..ebad5442d8bb 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -1896,30 +1896,21 @@ int __cgroup_bpf_run_filter_getsockopt(struct sock *sk, int level,
>   	if (max_optlen < 0)
>   		return max_optlen;
>   
> -	if (!retval) {
> -		/* If kernel getsockopt finished successfully,
> -		 * copy whatever was returned to the user back
> -		 * into our temporary buffer. Set optlen to the
> -		 * one that kernel returned as well to let
> -		 * BPF programs inspect the value.
> -		 */
> -
> -		if (get_user(ctx.optlen, optlen)) {
> -			ret = -EFAULT;
> -			goto out;
> -		}
> +	if (get_user(ctx.optlen, optlen)) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
>   
> -		if (ctx.optlen < 0) {
> -			ret = -EFAULT;
> -			goto out;
> -		}
> -		orig_optlen = ctx.optlen;
> +	if (ctx.optlen < 0) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +	orig_optlen = ctx.optlen;
>   
> -		if (copy_from_user(ctx.optval, optval,
> -				   min(ctx.optlen, max_optlen)) != 0) {
> -			ret = -EFAULT;
> -			goto out;
> -		}
> +	if (copy_from_user(ctx.optval, optval,
> +				min(ctx.optlen, max_optlen)) != 0) {
What is in optval that is useful to copy from if the kernel didn't handle the 
optname?

and there is no selftest also.

> +		ret = -EFAULT;
> +		goto out;
>   	}
>   
>   	lock_sock(sk);