netdev - Re: [PATCH bpf-next 1/3] bpf: Treat bpf_sk_lookup remote

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Message-ID: <20220318012219.wtrpgaawg4czsqcj@kafai-mbp.dhcp.thefacebook.com>
Date:   Thu, 17 Mar 2022 18:22:19 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        kernel-team@...udflare.com, Ilya Leoshkevich <iii@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH bpf-next 1/3] bpf: Treat bpf_sk_lookup remote_port as a
 2-byte field

On Thu, Mar 17, 2022 at 05:58:24PM +0100, Jakub Sitnicki wrote:
> In commit 9a69e2b385f4 ("bpf: Make remote_port field in struct
> bpf_sk_lookup 16-bit wide") the remote_port field has been split up and
> re-declared from u32 to be16.
> 
> However, the accompanying changes to the context access converter have not
> been well thought through when it comes big-endian platforms.
> 
> Today 2-byte wide loads from offsetof(struct bpf_sk_lookup, remote_port)
> are handled as narrow loads from a 4-byte wide field.
> 
> This by itself is not enough to create a problem, but when we combine
> 
>  1. 32-bit wide access to ->remote_port backed by a 16-wide wide load, with
>  2. inherent difference between litte- and big-endian in how narrow loads
>     need have to be handled (see bpf_ctx_narrow_access_offset),
> 
> we get inconsistent results for a 2-byte loads from &ctx->remote_port on LE
> and BE architectures. This in turn makes BPF C code for the common case of
> 2-byte load from ctx->remote_port not portable.
> 
> To rectify it, inform the context access converter that remote_port is
> 2-byte wide field, and only 1-byte loads need to be treated as narrow
> loads.
> 
> At the same time, we special-case the 4-byte load from &ctx->remote_port to
> continue handling it the same way as do today, in order to keep the
> existing BPF programs working.
> 
> Fixes: 9a69e2b385f4 ("bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide")
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  net/core/filter.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 03655f2074ae..9b1e453baf6d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -10989,13 +10989,24 @@ static bool sk_lookup_is_valid_access(int off, int size,
>  	case bpf_ctx_range(struct bpf_sk_lookup, local_ip4):
>  	case bpf_ctx_range_till(struct bpf_sk_lookup, remote_ip6[0], remote_ip6[3]):
>  	case bpf_ctx_range_till(struct bpf_sk_lookup, local_ip6[0], local_ip6[3]):
> -	case offsetof(struct bpf_sk_lookup, remote_port) ...
> -	     offsetof(struct bpf_sk_lookup, local_ip4) - 1:
>  	case bpf_ctx_range(struct bpf_sk_lookup, local_port):
>  	case bpf_ctx_range(struct bpf_sk_lookup, ingress_ifindex):
>  		bpf_ctx_record_field_size(info, sizeof(__u32));
>  		return bpf_ctx_narrow_access_ok(off, size, sizeof(__u32));
>  
> +	case bpf_ctx_range(struct bpf_sk_lookup, remote_port):
> +		/* Allow 4-byte access to 2-byte field for backward compatibility */
> +		if (size == sizeof(__u32))
> +			return off == offsetof(struct bpf_sk_lookup, remote_port);
nit. The bad "off" value should have been rejected earlier in the
"if (off % size != 0)" check?

> +		bpf_ctx_record_field_size(info, sizeof(__be16));
> +		return bpf_ctx_narrow_access_ok(off, size, sizeof(__be16));
> +
> +	case offsetofend(struct bpf_sk_lookup, remote_port) ...
> +	     offsetof(struct bpf_sk_lookup, local_ip4) - 1:
> +		/* Allow access to zero padding for backward compatibility */
> +		bpf_ctx_record_field_size(info, sizeof(__u16));
> +		return bpf_ctx_narrow_access_ok(off, size, sizeof(__u16));
> +
>  	default:
>  		return false;
>  	}
> @@ -11077,6 +11088,11 @@ static u32 sk_lookup_convert_ctx_access(enum bpf_access_type type,
>  						     sport, 2, target_size));
>  		break;
>  
> +	case offsetofend(struct bpf_sk_lookup, remote_port):
> +		*target_size = 2;
> +		*insn++ = BPF_MOV32_IMM(si->dst_reg, 0);
> +		break;
> +
>  	case offsetof(struct bpf_sk_lookup, local_port):
>  		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
>  				      bpf_target_off(struct bpf_sk_lookup_kern,
> -- 
> 2.35.1
>