SystemTap bpf begin probe doesn't work

[Eytan Naim]

Hi,

I'm trying to probe a userspace process with a 'begin' probe that doesn't
seem to do anything

begin.stp:

probe process("a.out").begin {
   printf("%s %d\n", execname(), pid())
}


stp output:

[root@RHEL8 ~]# stap --bpf    -v ~/begin.stp
Pass 1: parsed user script and 56 library scripts using
203356virt/48420res/12256shr/36024data kb, in 100usr/10sys/114real ms.
Pass 2: analyzed script: 2 probes, 3 functions, 0 embeds, 1 global
using 204676virt/50128res/12392shr/37344data kb, in 10usr/10sys/11real
ms.
Pass 3: pass skipped for stapbpf runtime in 0usr/0sys/0real ms.
Pass 4: compiled BPF into "stap_30984.bo" in 0usr/0sys/2real ms.
Pass 5: starting run.

When changing the probe to the main() function it seems to work:

function.stp

probe process("a.out").function("main") {
   printf("%s %d\n", execname(), pid())
}

stap output:

[root@RHEL8 ~]# stap --bpf    -v ~/222.stp
Pass 1: parsed user script and 56 library scripts using
203356virt/48364res/12200shr/36024data kb, in 290usr/50sys/404real ms.
Pass 2: analyzed script: 2 probes, 3 functions, 0 embeds, 1 global
using 204676virt/50996res/13068shr/37344data kb, in 20usr/0sys/25real
ms.
Pass 3: pass skipped for stapbpf runtime in 0usr/0sys/0real ms.
Pass 4: compiled BPF into "stap_31782.bo" in 0usr/0sys/3real ms.
Pass 5: starting run.
a.out 31806
a.out 31821
a.out 31827
a.out 31831

stap version

[root@RHEL8 ~]# stap --version
Systemtap translator/driver (version 4.2/0.178, rpm 4.2-6.el8)
Copyright (C) 2005-2019 Red Hat, Inc. and others
This is free software; see the source for copying conditions.
tested kernel versions: 2.6.32 ... 5.4-rc6
enabled features: AVAHI BOOST_STRING_REF DYNINST BPF JAVA PYTHON3
LIBRPM LIBSQLITE3 LIBVIRT LIBXML2 NLS NSS READLINE

Needless to say that using the default backend engine works.
Thanks,
Eytan.