Re: [PATCH] arm64: bpf: fix div-by-zero case

[Shi, Yang]

On 11/3/2015 10:56 PM, Zi Shen Lim wrote:
> In the case of division by zero in a BPF program:
> 	A = A / X;  (X == 0)
> the expected behavior is to terminate with return value 0.
>
> This is confirmed by the test case introduced in commit 86bf1721b226
> ("test_bpf: add tests checking that JIT/interpreter sets A and X to 0.").
>
> Reported-by: Shi, Yang <yang.shi@xxxxxxxxxx>

Thanks for coming up with the fix promptly.
s/Shi, Yang/Yang Shi

Tested with the latest 4.3 kernel.
Tested-by: Yang Shi <yang.shi@xxxxxxxxxx>

Yang

> CC: Xi Wang <xi.wang@xxxxxxxxx>
> CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> CC: Catalin Marinas <catalin.marinas@xxxxxxx>
> CC: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
> Cc: <stable@xxxxxxxxxxxxxxx> # 3.18+
> Signed-off-by: Zi Shen Lim <zlim.lnx@xxxxxxxxx>
> ---
>   arch/arm64/net/bpf_jit.h      |  3 ++-
>   arch/arm64/net/bpf_jit_comp.c | 37 +++++++++++++++++++++++++------------
>   2 files changed, 27 insertions(+), 13 deletions(-)
>
> diff --git a/arch/arm64/net/bpf_jit.h b/arch/arm64/net/bpf_jit.h
> index 98a26ce..aee5637 100644
> --- a/arch/arm64/net/bpf_jit.h
> +++ b/arch/arm64/net/bpf_jit.h
> @@ -1,7 +1,7 @@
>   /*
>    * BPF JIT compiler for ARM64
>    *
> - * Copyright (C) 2014 Zi Shen Lim <zlim.lnx@xxxxxxxxx>
> + * Copyright (C) 2014-2015 Zi Shen Lim <zlim.lnx@xxxxxxxxx>
>    *
>    * This program is free software; you can redistribute it and/or modify
>    * it under the terms of the GNU General Public License version 2 as
> @@ -35,6 +35,7 @@
>   	aarch64_insn_gen_comp_branch_imm(0, offset, Rt, A64_VARIANT(sf), \
>   		AARCH64_INSN_BRANCH_COMP_##type)
>   #define A64_CBZ(sf, Rt, imm19) A64_COMP_BRANCH(sf, Rt, (imm19) << 2, ZERO)
> +#define A64_CBNZ(sf, Rt, imm19) A64_COMP_BRANCH(sf, Rt, (imm19) << 2, NONZERO)
>
>   /* Conditional branch (immediate) */
>   #define A64_COND_BRANCH(cond, offset) \
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index c047598..9ae6f23 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -1,7 +1,7 @@
>   /*
>    * BPF JIT compiler for ARM64
>    *
> - * Copyright (C) 2014 Zi Shen Lim <zlim.lnx@xxxxxxxxx>
> + * Copyright (C) 2014-2015 Zi Shen Lim <zlim.lnx@xxxxxxxxx>
>    *
>    * This program is free software; you can redistribute it and/or modify
>    * it under the terms of the GNU General Public License version 2 as
> @@ -225,6 +225,17 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
>   	u8 jmp_cond;
>   	s32 jmp_offset;
>
> +#define check_imm(bits, imm) do {				\
> +	if ((((imm) > 0) && ((imm) >> (bits))) ||		\
> +	    (((imm) < 0) && (~(imm) >> (bits)))) {		\
> +		pr_info("[%2d] imm=%d(0x%x) out of range\n",	\
> +			i, imm, imm);				\
> +		return -EINVAL;					\
> +	}							\
> +} while (0)
> +#define check_imm19(imm) check_imm(19, imm)
> +#define check_imm26(imm) check_imm(26, imm)
> +
>   	switch (code) {
>   	/* dst = src */
>   	case BPF_ALU | BPF_MOV | BPF_X:
> @@ -258,8 +269,21 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
>   		break;
>   	case BPF_ALU | BPF_DIV | BPF_X:
>   	case BPF_ALU64 | BPF_DIV | BPF_X:
> +	{
> +		const u8 r0 = bpf2a64[BPF_REG_0];
> +
> +		/* if (src == 0) return 0 */
> +		jmp_offset = 3; /* skip ahead to else path */
> +		check_imm19(jmp_offset);
> +		emit(A64_CBNZ(is64, src, jmp_offset), ctx);
> +		emit(A64_MOVZ(1, r0, 0, 0), ctx);
> +		jmp_offset = epilogue_offset(ctx);
> +		check_imm26(jmp_offset);
> +		emit(A64_B(jmp_offset), ctx);
> +		/* else */
>   		emit(A64_UDIV(is64, dst, dst, src), ctx);
>   		break;
> +	}
>   	case BPF_ALU | BPF_MOD | BPF_X:
>   	case BPF_ALU64 | BPF_MOD | BPF_X:
>   		ctx->tmp_used = 1;
> @@ -393,17 +417,6 @@ emit_bswap_uxt:
>   		emit(A64_ASR(is64, dst, dst, imm), ctx);
>   		break;
>
> -#define check_imm(bits, imm) do {				\
> -	if ((((imm) > 0) && ((imm) >> (bits))) ||		\
> -	    (((imm) < 0) && (~(imm) >> (bits)))) {		\
> -		pr_info("[%2d] imm=%d(0x%x) out of range\n",	\
> -			i, imm, imm);				\
> -		return -EINVAL;					\
> -	}							\
> -} while (0)
> -#define check_imm19(imm) check_imm(19, imm)
> -#define check_imm26(imm) check_imm(26, imm)
> -
>   	/* JUMP off */
>   	case BPF_JMP | BPF_JA:
>   		jmp_offset = bpf2a64_offset(i + off, i, ctx);

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