Re: [PATCH bpf-next v4 09/10] bpf, x86: Jit support for nested bpf_prog_call

[Yonghong Song <yonghong.song@linux.dev>]

On 10/10/24 9:29 PM, Alexei Starovoitov wrote:
> On Thu, Oct 10, 2024 at 9:21 PM Yonghong Song <yonghong.song@linux.dev> wrote:
>>
>> On 10/10/24 1:53 PM, Alexei Starovoitov wrote:
>>> On Thu, Oct 10, 2024 at 10:59 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>>>>    static void emit_priv_frame_ptr(u8 **pprog, struct bpf_prog *bpf_prog,
>>>> -                               enum bpf_priv_stack_mode priv_stack_mode)
>>>> +                               enum bpf_priv_stack_mode priv_stack_mode,
>>>> +                               bool is_subprog, u8 *image, u8 *temp)
>>>>    {
>>>>           u32 orig_stack_depth = round_up(bpf_prog->aux->stack_depth, 8);
>>>>           u8 *prog = *pprog;
>>>>
>>>> -       if (priv_stack_mode == PRIV_STACK_ROOT_PROG)
>>>> -               emit_root_priv_frame_ptr(&prog, bpf_prog, orig_stack_depth);
>>>> -       else if (priv_stack_mode == PRIV_STACK_SUB_PROG && orig_stack_depth)
>>>> +       if (priv_stack_mode == PRIV_STACK_ROOT_PROG) {
>>>> +               int offs;
>>>> +               u8 *func;
>>>> +
>>>> +               if (!bpf_prog->aux->has_prog_call) {
>>>> +                       emit_root_priv_frame_ptr(&prog, bpf_prog, orig_stack_depth);
>>>> +               } else {
>>>> +                       EMIT1(0x57);            /* push rdi */
>>>> +                       if (is_subprog) {
>>>> +                               /* subprog may have up to 5 arguments */
>>>> +                               EMIT1(0x56);            /* push rsi */
>>>> +                               EMIT1(0x52);            /* push rdx */
>>>> +                               EMIT1(0x51);            /* push rcx */
>>>> +                               EMIT2(0x41, 0x50);      /* push r8 */
>>>> +                       }
>>>> +                       emit_mov_imm64(&prog, BPF_REG_1, (long) bpf_prog >> 32,
>>>> +                                      (u32) (long) bpf_prog);
>>>> +                       func = (u8 *)__bpf_prog_enter_recur_limited;
>>>> +                       offs = prog - temp;
>>>> +                       offs += x86_call_depth_emit_accounting(&prog, func, image + offs);
>>>> +                       emit_call(&prog, func, image + offs);
>>>> +                       if (is_subprog) {
>>>> +                               EMIT2(0x41, 0x58);      /* pop r8 */
>>>> +                               EMIT1(0x59);            /* pop rcx */
>>>> +                               EMIT1(0x5a);            /* pop rdx */
>>>> +                               EMIT1(0x5e);            /* pop rsi */
>>>> +                       }
>>>> +                       EMIT1(0x5f);            /* pop rdi */
>>>> +
>>>> +                       EMIT4(0x48, 0x83, 0xf8, 0x0);   /* cmp rax,0x0 */
>>>> +                       EMIT2(X86_JNE, num_bytes_of_emit_return() + 1);
>>>> +
>>>> +                       /* return if stack recursion has been reached */
>>>> +                       EMIT1(0xC9);    /* leave */
>>>> +                       emit_return(&prog, image + (prog - temp));
>>>> +
>>>> +                       /* cnt -= 1 */
>>>> +                       emit_alu_helper_1(&prog, BPF_ALU64 | BPF_SUB | BPF_K,
>>>> +                                         BPF_REG_0, 1);
>>>> +
>>>> +                       /* accum_stack_depth = cnt * subtree_stack_depth */
>>>> +                       emit_alu_helper_3(&prog, BPF_ALU64 | BPF_MUL | BPF_K, BPF_REG_0,
>>>> +                                         bpf_prog->aux->subtree_stack_depth);
>>>> +
>>>> +                       emit_root_priv_frame_ptr(&prog, bpf_prog, orig_stack_depth);
>>>> +
>>>> +                       /* r9 += accum_stack_depth */
>>>> +                       emit_alu_helper_2(&prog, BPF_ALU64 | BPF_ADD | BPF_X, X86_REG_R9,
>>>> +                                         BPF_REG_0);
>>> That's way too much asm for logic that can stay in C.
>>>
>>> bpf_trampoline_enter() should select __bpf_prog_enter_recur_limited()
>>> for appropriate prog_type/attach_type/etc.
>> The above jit code not just for the main prog, but also for callback fn's
>> since callback fn could call bpf prog as well. So putting in bpf trampoline
>> not enough.
> callback can call the prog only if bpf_call_prog() kfunc exists
> and that's one more reason to avoid going that direction.

Okay, I will add verifier check to prevent bpf_call_prog() in callback functions.