bpf: Fix a few selftest failures due to llvm18 change

With latest upstream llvm18, the following test cases failed:
  $ ./test_progs -j
  torvalds#13/2    bpf_cookie/multi_kprobe_link_api:FAIL
  torvalds#13/3    bpf_cookie/multi_kprobe_attach_api:FAIL
  torvalds#13      bpf_cookie:FAIL
  torvalds#77      fentry_fexit:FAIL
  torvalds#78/1    fentry_test/fentry:FAIL
  torvalds#78      fentry_test:FAIL
  torvalds#82/1    fexit_test/fexit:FAIL
  torvalds#82      fexit_test:FAIL
  torvalds#112/1   kprobe_multi_test/skel_api:FAIL
  torvalds#112/2   kprobe_multi_test/link_api_addrs:FAIL
  ...
  torvalds#112     kprobe_multi_test:FAIL
  torvalds#356/17  test_global_funcs/global_func17:FAIL
  torvalds#356     test_global_funcs:FAIL

Further analysis shows llvm upstream patch [1] is responsible
for the above failures. For example, for function bpf_fentry_test7()
in net/bpf/test_run.c, without [1], the asm code is:
  0000000000000400 <bpf_fentry_test7>:
     400: f3 0f 1e fa                   endbr64
     404: e8 00 00 00 00                callq   0x409 <bpf_fentry_test7+0x9>
     409: 48 89 f8                      movq    %rdi, %rax
     40c: c3                            retq
     40d: 0f 1f 00                      nopl    (%rax)
and with [1], the asm code is:
  0000000000005d20 <bpf_fentry_test7.specialized.1>:
    5d20: e8 00 00 00 00                callq   0x5d25 <bpf_fentry_test7.specialized.1+0x5>
    5d25: c3                            retq
and <bpf_fentry_test7.specialized.1> is called instead of <bpf_fentry_test7>
and this caused test failures for torvalds#13/torvalds#77 etc. except torvalds#356.

For test case torvalds#356/17, with [1] (progs/test_global_func17.c)),
the main prog looks like:
  0000000000000000 <global_func17>:
       0:       b4 00 00 00 2a 00 00 00 w0 = 0x2a
       1:       95 00 00 00 00 00 00 00 exit
which passed verification while the test itself expects a verification
failure.

Let us add 'barrier_var' style asm code in both places to prevent
function specialization which caused selftests failure.

  [1] llvm/llvm-project#72903

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

net/bpf/test_run.c

@@ -542,7 +542,7 @@ struct bpf_fentry_test_t {
 
 int noinline bpf_fentry_test7(struct bpf_fentry_test_t *arg)
 {
-	asm volatile ("");
+	asm volatile ("": "+r"(arg));
 	return (long)arg;
 }
 

tools/testing/selftests/bpf/progs/test_global_func17.c

@@ -5,6 +5,7 @@
 
 __noinline int foo(int *p)
 {
+	barrier_var(p);
 	return p ? (*p = 42) : 0;
 }