On 03/01/2019 12:39 PM, Arthur Fabre wrote:
> I can reproduce this on 4.19.0-3-amd64 both with, and without the JIT enabled.
>
> Dumping the "root" and "non-root" programs with bpftool,
> the subtraction instructions differ:
>
> "non-root":
> 0: (85) call bpf_ktime_get_ns#74944
> 1: (bf) r7 = r0
> 2: (85) call bpf_ktime_get_ns#74944
> 3: (bf) r6 = r0
> 4: (bf) r8 = r6
> 5: (b4) w11 = -1
> 6: (1f) r11 -= r8
> 7: (4f) r11 |= r8
> 8: (87) r11 = -r11
> 9: (c7) r11 s>>= 63
> 10: (5f) r8 &= r11
>
> "root":
> 0: (85) call bpf_ktime_get_ns#74944
> 1: (bf) r7 = r0
> 2: (85) call bpf_ktime_get_ns#74944
> 3: (bf) r6 = r0
> 4: (bf) r8 = r6
>
> The remainder of the instructions are for writing the results in the map,
> and the instructions are identical.
>
> I believe the extra instructions come from "fixup_bpf_calls" in the verifier:
>
> if (isneg)
> *patch++ = BPF_ALU64_IMM(BPF_MUL, off_reg, -1);
> *patch++ = BPF_MOV32_IMM(BPF_REG_AX, aux->alu_limit - 1);
> *patch++ = BPF_ALU64_REG(BPF_SUB, BPF_REG_AX, off_reg);
> *patch++ = BPF_ALU64_REG(BPF_OR, BPF_REG_AX, off_reg);
> *patch++ = BPF_ALU64_IMM(BPF_NEG, BPF_REG_AX, 0);
> *patch++ = BPF_ALU64_IMM(BPF_ARSH, BPF_REG_AX, 63);
> if (issrc) {
> *patch++ = BPF_ALU64_REG(BPF_AND, BPF_REG_AX,
> off_reg);
> insn->src_reg = BPF_REG_AX;
> } else {
> *patch++ = BPF_ALU64_REG(BPF_AND, off_reg,
> BPF_REG_AX);
> }
>
> This was introduced by "bpf: prevent out of bounds speculation on pointer
> arithmetic"
> (https://patchwork.ozlabs.org/patch/1039606/).
> I don't yet understand what's going on.
Hmm, thanks for the report, I'll take a look right away! There's no map
involved here it seems, so there shouldn't be such fixup.
Cheers,
Daniel
