netdev - Re: [PATCH bpf-next v2 3/3] selftests/bpf: Fix test for 4-byte load from dst

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Date:   Mon, 28 Feb 2022 22:22:07 -0800
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>
Subject: Re: [PATCH bpf-next v2 3/3] selftests/bpf: Fix test for 4-byte load
 from dst_port on big-endian

On Sun, Feb 27, 2022 at 09:27:57PM +0100, Jakub Sitnicki wrote:
> The check for 4-byte load from dst_port offset into bpf_sock is failing on
> big-endian architecture - s390. The bpf access converter rewrites the
> 4-byte load to a 2-byte load from sock_common at skc_dport offset, as shown
> below.
> 
>   * s390 / llvm-objdump -S --no-show-raw-insn
> 
>   00000000000002a0 <sk_dst_port__load_word>:
>         84:       r1 = *(u32 *)(r1 + 48)
>         85:       w0 = 1
>         86:       if w1 == 51966 goto +1 <LBB5_2>
>         87:       w0 = 0
>   00000000000002c0 <LBB5_2>:
>         88:       exit
> 
>   * s390 / bpftool prog dump xlated
> 
>   _Bool sk_dst_port__load_word(struct bpf_sock * sk):
>     35: (69) r1 = *(u16 *)(r1 +12)
>     36: (bc) w1 = w1
>     37: (b4) w0 = 1
>     38: (16) if w1 == 0xcafe goto pc+1
>     39: (b4) w0 = 0
>     40: (95) exit
> 
>   * s390 / llvm-objdump -S --no-show-raw-insn
x86_64

> 
>   00000000000002a0 <sk_dst_port__load_word>:
>         84:       r1 = *(u32 *)(r1 + 48)
>         85:       w0 = 1
>         86:       if w1 == 65226 goto +1 <LBB5_2>
>         87:       w0 = 0
>   00000000000002c0 <LBB5_2>:
>         88:       exit
> 
>   * x86_64 / bpftool prog dump xlated
> 
>   _Bool sk_dst_port__load_word(struct bpf_sock * sk):
>     33: (69) r1 = *(u16 *)(r1 +12)
>     34: (b4) w0 = 1
>     35: (16) if w1 == 0xfeca goto pc+1
>     36: (b4) w0 = 0
>     37: (95) exit
> 
> This leads to surprisings results. On big-endian platforms, the loaded
> value is as expected. The user observes no difference between a 4-byte load
> and 2-byte load. However, on little-endian platforms, the access conversion
> is not what would be expected, that is the result is left shifted after
> converting the value to the native byte order.
> 
> That said, 4-byte loads in BPF from sk->dst_port are not a use case we
> expect to see, now that the dst_port field is clearly declared as a u16.
> 
> Account for the quirky behavior of the access converter in the test case,
> so that the check passes on both endian variants.
> 
> Fixes: 8f50f16ff39d ("selftests/bpf: Extend verifier and bpf_sock tests for dst_port loads")
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  .../selftests/bpf/progs/test_sock_fields.c        | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/progs/test_sock_fields.c b/tools/testing/selftests/bpf/progs/test_sock_fields.c
> index 186fed1deaab..3dddc173070c 100644
> --- a/tools/testing/selftests/bpf/progs/test_sock_fields.c
> +++ b/tools/testing/selftests/bpf/progs/test_sock_fields.c
> @@ -256,10 +256,23 @@ int ingress_read_sock_fields(struct __sk_buff *skb)
>  	return CG_OK;
>  }
>  
> +/*
> + * NOTE: 4-byte load from bpf_sock at dst_port offset is quirky. The
> + * result is left shifted on little-endian architectures because the
> + * access is converted to a 2-byte load. The quirky behavior is kept
> + * for backward compatibility.
> + */
>  static __noinline bool sk_dst_port__load_word(struct bpf_sock *sk)
>  {
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +	const __u8 SHIFT = 16;
> +#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> +	const __u8 SHIFT = 0;
> +#else
> +#error "Unrecognized __BYTE_ORDER__"
> +#endif
>  	__u32 *word = (__u32 *)&sk->dst_port;
> -	return word[0] == bpf_htonl(0xcafe0000);
> +	return word[0] == bpf_htonl(0xcafe << SHIFT);
I believe it should be fine.  It is the behavior even before
commit 4421a582718a ("bpf: Make dst_port field in struct bpf_sock 16-bit wide") ?

btw, is it the same as testing "return word[0] == bpf_hton's'(0xcafe);"

>  }
>  
>  static __noinline bool sk_dst_port__load_half(struct bpf_sock *sk)
> -- 
> 2.35.1
>