linux-kernel - Re: [PATCH v1 2/2] LoongArch: BPF: Fix unconditional bswap instructions

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Date:   Sat, 9 Dec 2023 14:06:28 +0800
From:   Hengqi Chen <hengqi.chen@...il.com>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...nel.org>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] LoongArch: BPF: Fix unconditional bswap instructions

On Thu, Dec 7, 2023 at 11:30 AM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> We can see that "bswap32: Takes an unsigned 32-bit number in either
> big- or little-endian format and returns the equivalent number with
> the same bit width but opposite endianness" in BPF Instruction Set
> Specification, so it should clear the upper 32 bits in the case 32
> for BPF_ALU and BPF_ALU64.
>
> [root@...ux fedora]# echo 1 > /proc/sys/net/core/bpf_jit_enable
> [root@...ux fedora]# modprobe test_bpf
>
> Before:
> test_bpf: #313 BSWAP 32: 0x0123456789abcdef -> 0xefcdab89 jited:1 ret 1460850314 != -271733879 (0x5712ce8a != 0xefcdab89)FAIL (1 times)
> test_bpf: #317 BSWAP 32: 0xfedcba9876543210 -> 0x10325476 jited:1 ret -1460850316 != 271733878 (0xa8ed3174 != 0x10325476)FAIL (1 times)
>
> After:
> test_bpf: #313 BSWAP 32: 0x0123456789abcdef -> 0xefcdab89 jited:1 4 PASS
> test_bpf: #317 BSWAP 32: 0xfedcba9876543210 -> 0x10325476 jited:1 4 PASS
>

Nice catch. I wasn't aware of test_bpf before. For the patch:

Acked-by: Hengqi Chen <hengqi.chen@...il.com>

> Fixes: 4ebf9216e7df ("LoongArch: BPF: Support unconditional bswap instructions")
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  arch/loongarch/net/bpf_jit.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
> index 8c907c2c42f7..da48398e8794 100644
> --- a/arch/loongarch/net/bpf_jit.c
> +++ b/arch/loongarch/net/bpf_jit.c
> @@ -774,8 +774,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
>                         break;
>                 case 32:
>                         emit_insn(ctx, revb2w, dst, dst);
> -                       /* zero-extend 32 bits into 64 bits */
> -                       emit_zext_32(ctx, dst, is32);
> +                       /* clear the upper 32 bits */
> +                       emit_zext_32(ctx, dst, true);
>                         break;
>                 case 64:
>                         emit_insn(ctx, revbd, dst, dst);
> --
> 2.42.0
>