linux-kernel - Re: [PATCH v1 1/2] LoongArch: BPF: Fix sign-extension mov instructions

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Date:   Thu, 7 Dec 2023 12:12:10 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Hengqi Chen <hengqi.chen@...il.com>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] LoongArch: BPF: Fix sign-extension mov instructions

On Thu, Dec 7, 2023 at 11:30 AM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> We can see that "Short form of movsx, dst_reg = (s8,s16,s32)src_reg"
> in include/linux/filter.h, additionally, for BPF_ALU64 the value of
> the destination register is unchanged whereas for BPF_ALU the upper
> 32 bits of the destination register are zeroed, so it should clear
> the upper 32 bits for BPF_ALU.
>
> [root@...ux fedora]# echo 1 > /proc/sys/net/core/bpf_jit_enable
> [root@...ux fedora]# modprobe test_bpf
>
> Before:
> test_bpf: #81 ALU_MOVSX | BPF_B jited:1 ret 2 != 1 (0x2 != 0x1)FAIL (1 times)
> test_bpf: #82 ALU_MOVSX | BPF_H jited:1 ret 2 != 1 (0x2 != 0x1)FAIL (1 times)
>
> After:
> test_bpf: #81 ALU_MOVSX | BPF_B jited:1 6 PASS
> test_bpf: #82 ALU_MOVSX | BPF_H jited:1 6 PASS
>
> By the way, the bpf selftest case "./test_progs -t verifier_movsx" can
> also be fixed with this patch.
Hmmm, it is a little strange that privileged verifier_movsx has no problem.

Huacai

>
> Fixes: f48012f16150 ("LoongArch: BPF: Support sign-extension mov instructions")
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  arch/loongarch/net/bpf_jit.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
> index 169ff8b3915e..8c907c2c42f7 100644
> --- a/arch/loongarch/net/bpf_jit.c
> +++ b/arch/loongarch/net/bpf_jit.c
> @@ -480,10 +480,12 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
>                 case 8:
>                         move_reg(ctx, t1, src);
>                         emit_insn(ctx, extwb, dst, t1);
> +                       emit_zext_32(ctx, dst, is32);
>                         break;
>                 case 16:
>                         move_reg(ctx, t1, src);
>                         emit_insn(ctx, extwh, dst, t1);
> +                       emit_zext_32(ctx, dst, is32);
>                         break;
>                 case 32:
>                         emit_insn(ctx, addw, dst, src, LOONGARCH_GPR_ZERO);
> --
> 2.42.0
>
>