[Bug bpf/27031] New: bpf: @var() not supported?

["agentzh at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27031

            Bug ID: 27031
           Summary: bpf: @var() not supported?
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: bpf
          Assignee: systemtap at sourceware dot org
          Reporter: agentzh at gmail dot com
  Target Milestone: ---

I've noticed that @var() does not work at all when tracing userland processes,
as demonstrated below on Fedora 32 x86_64 using the latest master branch of the
systemtap repo:

```
[agentzh@fed32-min ~/git/systemtap 1 (master)]$ cat a.c
long a = 0xdeadbeef;

int main (void) {
    return 0;
}

[agentzh@fed32-min ~/git/systemtap 0 (master)]$ gcc -g a.c -o /tmp/a.out

[agentzh@fed32-min ~/git/systemtap 0 (master)]$ /opt/stap/bin/stap --bpf -c
/tmp/a.out -e 'probe process("/tmp/a.out").function("main") { println("a: ",
@var("a")) exit() }'
semantic error: bpf translation failure: invalid or missing conversion
specifier: keyword at <input>:1:1
        source: probe process("/tmp/a.out").function("main") { println("a: ",
@var("a")) exit() }
                ^

Pass 4: compilation failed.  [man error::pass4]


[agentzh@fed32-min ~/git/systemtap 0 (master)]$ /opt/stap/bin/stap --bpf -c
/tmp/a.out -e 'probe process("/tmp/a.out").function("main") { println("a: ",
@var("a", "/tmp/a.out")) exit() }'
semantic error: bpf translation failure: invalid or missing conversion
specifier: keyword at <input>:1:1
        source: probe process("/tmp/a.out").function("main") { println("a: ",
@var("a", "/tmp/a.out")) exit() }
                ^

Pass 4: compilation failed.  [man error::pass4]

[agentzh@fed32-min ~/git/systemtap 0 (master)]$ /opt/stap/bin/stap -V
Systemtap translator/driver (version 4.5/0.182, commit
release-4.4-24-g3e0d449fe427 + changes)
Copyright (C) 2005-2020 Red Hat, Inc. and others
This is free software; see the source for copying conditions.
tested kernel versions: 2.6.32 ... 5.10.0-rc
enabled features: BPF NLS

[agentzh@fed32-min ~/git/systemtap 0 (master)]$ cat /proc/version
Linux version 5.9.11-100.fc32.x86_64
(mockbuild@bkernel01.iad2.fedoraproject.org) (gcc (GCC) 10.2.1 20201016 (Red
Hat 10.2.1-6), GNU ld version 2.34-6.fc32) #1 SMP Tue Nov 24 19:16:53 UTC 2020
```

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