> 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?
Thanks,
Song
> + start_of_ldx = prog;
> + }
> emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
> if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
> struct exception_table_entry *ex;
> u8 *_insn = image + proglen;
> s64 delta;
>
> + /* populate jmp_offset for JMP above */
> + start_of_ldx[-1] = prog - start_of_ldx;
> +
> if (!bpf_prog->aux->extable)
> break;
>
> --
> 2.24.1
>
