Re: [PATCH] bpf: selftests: test_verifier: mask bpf_csum_diff() return value to 16 bits

From: Yonghong Song <yhs@fb.com>
To: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>, <bpf@vger.kernel.org>
Cc: <daniel@iogearbox.net>, <toke@redhat.com>
Subject: Re: [PATCH] bpf: selftests: test_verifier: mask bpf_csum_diff() return value to 16 bits
Date: Sun, 28 Feb 2021 21:18:34 -0800	[thread overview]
Message-ID: <acf00517-6129-869b-cd2a-03715de5fc61@fb.com> (raw)
In-Reply-To: <20210228103017.320240-1-yauheni.kaliuta@redhat.com>

On 2/28/21 2:30 AM, Yauheni Kaliuta wrote:
> The verifier test labelled "valid read map access into a read-only array
> 2" calls the bpf_csum_diff() helper and checks its return value.
> However, architecture implementations of csum_partial() (which is what
> the helper uses) differ in whether they fold the return value to 16 bit
> or not. For example, x86 version has:
> 
> 	if (unlikely(odd)) {
> 		result = from32to16(result);
> 		result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
> 	}
> 
> while generic lib/checksum.c does:
> 
> 	result = from32to16(result);
> 	if (odd)
> 		result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
> 
> This makes the helper return different values on different
> architectures, breaking the test on non-x86. To fix this, add an

I remember there is a previous discussion for this issue, csum_diff()
returns different results for different architecture? Daniel?
Any conclusion how to deal with this?

> additional instruction to always mask the return value to 16 bits, and
> update the expected return value accordingly.
> 
> Fixes: fb2abb73e575 ("bpf, selftest: test {rd, wr}only flags and direct value access")
> Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
> ---
>   tools/testing/selftests/bpf/verifier/array_access.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/verifier/array_access.c b/tools/testing/selftests/bpf/verifier/array_access.c
> index bed53b561e04..1b138cd2b187 100644
> --- a/tools/testing/selftests/bpf/verifier/array_access.c
> +++ b/tools/testing/selftests/bpf/verifier/array_access.c
> @@ -250,12 +250,13 @@
>   	BPF_MOV64_IMM(BPF_REG_5, 0),
>   	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
>   		     BPF_FUNC_csum_diff),
> +	BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xffff),
>   	BPF_EXIT_INSN(),
>   	},
>   	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
>   	.fixup_map_array_ro = { 3 },
>   	.result = ACCEPT,
> -	.retval = -29,
> +	.retval = 65507,
>   },
>   {
>   	"invalid write map access into a read-only array 1",
> 