Re: [PATCH bpf-next v9 02/10] bpf: Return false for bpf_prog_check_recur() default case

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

On 11/4/24 6:53 PM, Yonghong Song wrote:
>
> On 11/4/24 5:55 PM, Alexei Starovoitov wrote:
>> On Mon, Nov 4, 2024 at 5:35 PM Yonghong Song 
>> <yonghong.song@linux.dev> wrote:
>>>
>>> On 11/4/24 5:21 PM, Alexei Starovoitov wrote:
>>>> On Mon, Nov 4, 2024 at 11:35 AM Yonghong Song 
>>>> <yonghong.song@linux.dev> wrote:
>>>>> The bpf_prog_check_recur() funciton is currently used by trampoline
>>>>> and tracing programs (also using trampoline) to check whether a
>>>>> particular prog supports recursion checking or not. The default case
>>>>> (non-trampoline progs) return true in the current implementation.
>>>>>
>>>>> Let us make the non-trampoline prog recursion check return false
>>>>> instead. It does not impact any existing use cases and allows the
>>>>> function to be used outside the trampoline context in the next patch.
>>>> Does not impact ?! But it does.
>>>> This patch removes recursion check from fentry progs.
>>>> This cannot be right.
>>> The original bpf_prog_check_recur() implementation:
>>>
>>> static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
>>> {
>>>           switch (resolve_prog_type(prog)) {
>>>           case BPF_PROG_TYPE_TRACING:
>>>                   return prog->expected_attach_type != BPF_TRACE_ITER;
>>>           case BPF_PROG_TYPE_STRUCT_OPS:
>>>           case BPF_PROG_TYPE_LSM:
>>>                   return false;
>>>           default:
>>>                   return true;
>>>           }
>>> }
>>>
>>> fentry prog is a TRACING prog, so it is covered. Did I miss anything?
>> I see. This is way too subtle.
>> You're correct that fentry is TYPE_TRACING,
>> so it could have "worked" if it was used to build trampolines only.
>>
>> But this helper is called for other prog types:
>>
>>          case BPF_FUNC_task_storage_get:
>>                  if (bpf_prog_check_recur(prog))
>>                          return &bpf_task_storage_get_recur_proto;
>>                  return &bpf_task_storage_get_proto;
>>
>> so it's still not correct, but for a different reason.
>
> There are four uses for func bpf_prog_check_recur() in kernel based on 
> cscope: 0 kernel/bpf/trampoline.c bpf_trampoline_enter 1053 if 
> (bpf_prog_check_recur(prog)) 1 kernel/bpf/trampoline.c 
> bpf_trampoline_exit 1068 if (bpf_prog_check_recur(prog)) 2 
> kernel/trace/bpf_trace.c bpf_tracing_func_proto 1549 if 
> (bpf_prog_check_recur(prog)) 3 kernel/trace/bpf_trace.c 
> bpf_tracing_func_proto 1553 if (bpf_prog_check_recur(prog)) The 2nd 
> and 3rd ones are in bpf_trace.c. 1444 static const struct 
> bpf_func_proto * 1445 bpf_tracing_func_proto(enum bpf_func_id func_id, 
> const struct bpf_prog *prog) 1446 { 1447 switch (func_id) { ... 1548 
> case BPF_FUNC_task_storage_get: 1549 if (bpf_prog_check_recur(prog)) 
> 1550 return &bpf_task_storage_get_recur_proto; 1551 return 
> &bpf_task_storage_get_proto; 1552 case BPF_FUNC_task_storage_delete: 
> 1553 if (bpf_prog_check_recur(prog)) 1554 return 
> &bpf_task_storage_delete_recur_proto; 1555 return 
> &bpf_task_storage_delete_proto; ... 1568 default: 1569 return 
> bpf_base_func_proto(func_id, prog); 1570 } 1571 } They are used for 
> tracing programs. So we should be safe here. But if you think that 
> changing bpf_proc_check_recur() and calling function 
> bpf_prog_check_recur() in bpf_enable_priv_stack() is too subtle, I can 
> go back to my original approach which makes all supported prog types 
> explicit in bpf_enable_priv_stack().

Sorry. Format issue again. The below is a better format:

There are four uses for func bpf_prog_check_recur() in kernel based on cscope:

0 kernel/bpf/trampoline.c bpf_trampoline_enter 1053 if (bpf_prog_check_recur(prog))
1 kernel/bpf/trampoline.c bpf_trampoline_exit 1068 if (bpf_prog_check_recur(prog))
2 kernel/trace/bpf_trace.c bpf_tracing_func_proto 1549 if (bpf_prog_check_recur(prog))
3 kernel/trace/bpf_trace.c bpf_tracing_func_proto 1553 if (bpf_prog_check_recur(prog))

The 2nd and 3rd ones are in bpf_trace.c.

1444 static const struct bpf_func_proto *
1445 bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
1446 {
1447     switch (func_id) {
...
1548     case BPF_FUNC_task_storage_get:
1549         if (bpf_prog_check_recur(prog))
1550             return &bpf_task_storage_get_recur_proto;
1551         return &bpf_task_storage_get_proto;
1552     case BPF_FUNC_task_storage_delete:
1553         if (bpf_prog_check_recur(prog))
1554             return &bpf_task_storage_delete_recur_proto;
1555         return &bpf_task_storage_delete_proto;
...
1568     default:
1569         return bpf_base_func_proto(func_id, prog);
1570     }
1571 }
  
They are used for tracing programs. So we should be safe here. But if you think that
changing bpf_proc_check_recur() and calling function bpf_prog_check_recur()
in bpf_enable_priv_stack() is too subtle, I can go back to my original approach
which makes all supported prog types explicit in bpf_enable_priv_stack().