Error when using bpf_strncmp

[BrianBaboch]

Hello,

I'm having problems when using bpf_strncmp, I have a simple ebpf program using BCC, that program uses bpf_strncmp to compare the task name to a string.
So I do the following:

    char my_com[] = "cat";
    struct task_struct *task = (struct task_struct *)bpf_get_current_task_btf();
    if (bpf_strncmp(comm, sizeof(my_com), my_com) == 0){
        bpf_trace_printk("Cat");
    }

If I use this I get a permission denied error that I can't seem to understand while, if I use strncmp it works as expected

    char my_com[] = "cat";
    struct task_struct *task = (struct task_struct *)bpf_get_current_task_btf();
    if (strncmp(comm, my_com, sizeof(my_com) -1) == 0){
        bpf_trace_printk("Cat");
    }

The error:

    b.attach_raw_tracepoint(tp="sys_enter", fn_name="hello")
  File "/usr/lib/python3/dist-packages/bcc/__init__.py", line 1061, in attach_raw_tracepoint
    fn = self.load_func(fn_name, BPF.RAW_TRACEPOINT)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/bcc/__init__.py", line 526, in load_func
    raise Exception("Failed to load BPF program %s: %s" %
Exception: Failed to load BPF program b'hello': Permission denied

In the ebpf compiled code I have the following:

R3 type=fp expected=map_value

However, the bpf_strncmp takes char * as mentioned in the doc so I don't understand the problem

[yonghong-song]

See error

   R3 type=fp expected=map_value

Here,

  char my_com[] = "cat";

has my_com in stack (fp) and verifier does not like it.

The kernel specification:

static const struct bpf_func_proto bpf_strncmp_proto = {
        .func           = bpf_strncmp,
        .gpl_only       = false,
        .ret_type       = RET_INTEGER,
        .arg1_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
        .arg2_type      = ARG_CONST_SIZE,
        .arg3_type      = ARG_PTR_TO_CONST_STR,
};

The third argument needs to be a ptr to const str.

Try

    char my_com = "cat";

to see whether it works or not.