> On Feb 2, 2021, at 6:19 PM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Feb 03, 2021 at 12:56:39AM +0000, Song Liu wrote:
>>
>>
>>> On Feb 1, 2021, at 9:38 PM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
>>>
>>> From: Alexei Starovoitov <ast@xxxxxxxxxx>
>>>
>>> PTR_TO_BTF_ID registers contain either kernel pointer or NULL.
>>> Emit the NULL check explicitly by JIT instead of going into
>>> do_user_addr_fault() on NULL deference.
>>>
>>> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
>>> ---
>>> arch/x86/net/bpf_jit_comp.c | 19 +++++++++++++++++++
>>> 1 file changed, 19 insertions(+)
>>>
>>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>>> index b7a2911bda77..a3dc3bd154ac 100644
>>> --- a/arch/x86/net/bpf_jit_comp.c
>>> +++ b/arch/x86/net/bpf_jit_comp.c
>>> @@ -930,6 +930,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
>>> u32 dst_reg = insn->dst_reg;
>>> u32 src_reg = insn->src_reg;
>>> u8 b2 = 0, b3 = 0;
>>> + u8 *start_of_ldx;
>>> s64 jmp_offset;
>>> u8 jmp_cond;
>>> u8 *func;
>>> @@ -1278,12 +1279,30 @@ st: if (is_imm8(insn->off))
>>> case BPF_LDX | BPF_PROBE_MEM | BPF_W:
>>> case BPF_LDX | BPF_MEM | BPF_DW:
>>> case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
>>> + if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
>>> + /* test src_reg, src_reg */
>>> + maybe_emit_mod(&prog, src_reg, src_reg, true); /* always 1 byte */
>>> + EMIT2(0x85, add_2reg(0xC0, src_reg, src_reg));
>>> + /* jne start_of_ldx */
>>> + EMIT2(X86_JNE, 0);
>>> + /* xor dst_reg, dst_reg */
>>> + emit_mov_imm32(&prog, false, dst_reg, 0);
>>> + /* jmp byte_after_ldx */
>>> + EMIT2(0xEB, 0);
>>> +
>>> + /* populate jmp_offset for JNE above */
>>> + temp[4] = prog - temp - 5 /* sizeof(test + jne) */;
>>
>> IIUC, this case only happens for i == 1 in the loop? If so, can we use temp[5(?)]
>> instead of start_of_ldx?
>
> I don't understand the question, but let me try anyway :)
> temp is a buffer for single instruction.
> prog=temp; for every loop iteration (not only i == 1)
Thanks for the explanation. I misunderstood how we use prog in the loop.
> temp[4] is second byte in JNE instruction as the comment says.
> temp[5] is a byte after JNE. It's a first byte of XOR.
> That XOR is variable length instruction.
> Hence while emitting JNE we don't know the target offset in JNE and just use 0.
> So temp[4] assignment populates with actual offset, since now we know the size
> of XOR.
And after reading emit_ldx() more carefully, I agree that introducing
start_of_ldx would simplify the logic here.
Acked-by: Song Liu <songliubraving@xxxxxx>
