selftests/bpf: Fix dynptr/test_dynptr_is_null

With latest llvm17, dynptr/test_dynptr_is_null subtest failed in my testing
VM. The failure log looks like below:

  All error logs:
  tester_init:PASS:tester_log_buf 0 nsec
  process_subtest:PASS:obj_open_mem 0 nsec
  process_subtest:PASS:Can't alloc specs array 0 nsec
  verify_success:PASS:dynptr_success__open 0 nsec
  verify_success:PASS:bpf_object__find_program_by_name 0 nsec
  verify_success:PASS:dynptr_success__load 0 nsec
  verify_success:PASS:bpf_program__attach 0 nsec
  verify_success:FAIL:err unexpected err: actual 4 != expected 0
  #65/9    dynptr/test_dynptr_is_null:FAIL

The error happens for bpf prog test_dynptr_is_null in dynptr_success.c:

        if (bpf_dynptr_is_null(&ptr2)) {
                err = 4;
                goto exit;
        }

The bpf_dynptr_is_null(&ptr) unexpectedly returned a non-zero value and
the control went to the error path. Digging further, I found the root cause
is due to function signature difference between kernel and user space.

In kernel, we have ...

  __bpf_kfunc bool bpf_dynptr_is_null(struct bpf_dynptr_kern *ptr)

... while in bpf_kfuncs.h we have:

  extern int bpf_dynptr_is_null(const struct bpf_dynptr *ptr) __ksym;

The kernel bpf_dynptr_is_null disasm code:

  ffffffff812f1a90 <bpf_dynptr_is_null>:
  ffffffff812f1a90: f3 0f 1e fa           endbr64
  ffffffff812f1a94: 0f 1f 44 00 00        nopl    (%rax,%rax)
  ffffffff812f1a99: 53                    pushq   %rbx
  ffffffff812f1a9a: 48 89 fb              movq    %rdi, %rbx
  ffffffff812f1a9d: e8 ae 29 17 00        callq   0xffffffff81464450 <__asan_load8_noabort>
  ffffffff812f1aa2: 48 83 3b 00           cmpq    $0x0, (%rbx)
  ffffffff812f1aa6: 0f 94 c0              sete    %al
  ffffffff812f1aa9: 5b                    popq    %rbx
  ffffffff812f1aaa: c3                    retq

Note that only 1-byte register %al is set and the other 7-bytes are not
touched. In bpf program, the asm code for the above bpf_dynptr_is_null(&ptr2):

       266:       85 10 00 00 ff ff ff ff call -0x1
       267:       b4 01 00 00 04 00 00 00 w1 = 0x4
       268:       16 00 03 00 00 00 00 00 if w0 == 0x0 goto +0x3 <LBB9_8>

Basically, 4-byte subregister is tested. This might cause error as the value
other than the lowest byte might not be 0.

This patch fixed the issue by using the identical func prototype across kernel
and selftest user space. The fixed bpf asm code:

       267:       85 10 00 00 ff ff ff ff call -0x1
       268:       54 00 00 00 01 00 00 00 w0 &= 0x1
       269:       b4 01 00 00 04 00 00 00 w1 = 0x4
       270:       16 00 03 00 00 00 00 00 if w0 == 0x0 goto +0x3 <LBB9_8>

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230517040404.4023912-1-yhs@fb.com