netdev - Re: [PATCH bpf-next] arm64: bpf: zero upper bits after rev32

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Date: Thu, 21 Mar 2024 10:00:35 +0800
From: Xu Kuohai <xukuohai@...wei.com>
To: Xi Wang <xi.wang@...il.com>, Artem Savkov <asavkov@...hat.com>
CC: Catalin Marinas <catalin.marinas@....com>, Alexei Starovoitov
	<ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
	<andrii@...nel.org>, <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] arm64: bpf: zero upper bits after rev32

On 3/21/2024 12:15 AM, Xi Wang wrote:
> On Wed, Mar 13, 2024 at 7:02 AM Artem Savkov <asavkov@...hat.com> wrote:
>> Commit d63903bbc30c7 ("arm64: bpf: fix endianness conversion bugs")
>> added upper bits zeroing to byteswap operations, but it assumes they
>> will be already zeroed after rev32, which is not the case on some
>> systems at least:
>>
>> [ 9757.262607] test_bpf: #312 BSWAP 16: 0x0123456789abcdef -> 0xefcd jited:1 8 PASS
>> [ 9757.264435] test_bpf: #313 BSWAP 32: 0x0123456789abcdef -> 0xefcdab89 jited:1 ret 1460850314 != -271733879 (0x5712ce8a != 0xefcdab89)FAIL (1 times)
>> [ 9757.266260] test_bpf: #314 BSWAP 64: 0x0123456789abcdef -> 0x67452301 jited:1 8 PASS
>> [ 9757.268000] test_bpf: #315 BSWAP 64: 0x0123456789abcdef >> 32 -> 0xefcdab89 jited:1 8 PASS
>> [ 9757.269686] test_bpf: #316 BSWAP 16: 0xfedcba9876543210 -> 0x1032 jited:1 8 PASS
>> [ 9757.271380] test_bpf: #317 BSWAP 32: 0xfedcba9876543210 -> 0x10325476 jited:1 ret -1460850316 != 271733878 (0xa8ed3174 != 0x10325476)FAIL (1 times)
>> [ 9757.273022] test_bpf: #318 BSWAP 64: 0xfedcba9876543210 -> 0x98badcfe jited:1 7 PASS
>> [ 9757.274721] test_bpf: #319 BSWAP 64: 0xfedcba9876543210 >> 32 -> 0x10325476 jited:1 9 PASS
>>
>> Fixes: d63903bbc30c7 ("arm64: bpf: fix endianness conversion bugs")
> 
> This tag is not right.  It's unlikely that the bug has been around for 9 years.
> 
> Maybe you meant 1104247f3f979 ("bpf, arm64: Support unconditional bswap")?
>

Agree, thanks for pointing it out.

>> Signed-off-by: Artem Savkov <asavkov@...hat.com>
>> ---
>>   arch/arm64/net/bpf_jit_comp.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>> index c5b461dda4385..e86e5ba74dca2 100644
>> --- a/arch/arm64/net/bpf_jit_comp.c
>> +++ b/arch/arm64/net/bpf_jit_comp.c
>> @@ -944,7 +944,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
>>                          break;
>>                  case 32:
>>                          emit(A64_REV32(is64, dst, dst), ctx);
>> -                       /* upper 32 bits already cleared */
>> +                       /* zero-extend 32 bits into 64 bits */
>> +                       emit(A64_UXTW(is64, dst, dst), ctx);
> 
> The fix can pass the tests, but emitting an extra instruction is
> unnecessary as the bug applies only to unconditional bswap.
> 
>>                          break;
>>                  case 64:
>>                          emit(A64_REV64(dst, dst), ctx);
>> --
>> 2.44.0
>>
>