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    Subject[PATCH] arm64: bpf: fix div-by-zero case
    Date
    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@linaro.org>
    CC: Xi Wang <xi.wang@gmail.com>
    CC: Alexei Starovoitov <ast@plumgrid.com>
    CC: Catalin Marinas <catalin.marinas@arm.com>
    CC: linux-arm-kernel@lists.infradead.org
    CC: linux-kernel@vger.kernel.org
    Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
    Cc: <stable@vger.kernel.org> # 3.18+
    Signed-off-by: Zi Shen Lim <zlim.lnx@gmail.com>
    ---
    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@gmail.com>
    + * Copyright (C) 2014-2015 Zi Shen Lim <zlim.lnx@gmail.com>
    *
    * 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@gmail.com>
    + * Copyright (C) 2014-2015 Zi Shen Lim <zlim.lnx@gmail.com>
    *
    * 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);
    --
    1.9.1


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