[Bug tapsets/25290] New: process(EXE).begin may occasionally miss already-running target processes from EXE

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

https://sourceware.org/bugzilla/show_bug.cgi?id=25290

            Bug ID: 25290
           Summary: process(EXE).begin may occasionally miss
                    already-running target processes from EXE
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tapsets
          Assignee: systemtap at sourceware dot org
          Reporter: agentzh at gmail dot com
  Target Milestone: ---

I've noted that the process(EXE).begin probes may sometimes miss target
processes which are already running when staprun is started. To reproduce this:

1. Prepare a minimal a.c C program file:

```
#include <unistd.h>

int main (int argc, char** argv) {
    while (1) {
        usleep(1);
    }
    return 0;
}
```

Compile it like this:

    gcc -g a.c

which results in ./a.out generated. Run this program in a separate terminal
window like this:


    ./a.out

Keep this process running (it has an infinite loop so it will never exit by
itself).

2. Prepare a minimal a.stp script file:

```
global max_cnt = 5, cnt = 0;
global pids

probe process("/home/agentzh/a.out").begin {
    pids[pid()] = 1
}

probe process("/home/agentzh/a.out").end {
    delete pids[pid()];
}

probe process("/lib64/libc.so.6").function("usleep") {
    if (!pids[pid()])
        next;

    if (++cnt > max_cnt)
        exit();

    println("usleep(", @var("useconds"), ")");
}

probe timer.s(5) {
    warn("timer expired");
    exit();
}

probe begin {
    warn("Start tracing...")
}
```

And then compile this file to a kernel module file, usleep.ko:


    stap -p4 -m usleep a.stp


3. Run this usleep.ko module in a shell loop:


    ( while true; do echo ======; sudo staprun usleep.ko; sleep 0.1; done ) |&
tee a.txt

after a few minutes (be patient!), we should see "timer expired" messages from
the output file a.txt:

    $ grep -a timer a.txt
    WARNING: timer expired
    WARNING: timer expired
    WARNING: timer expired
    WARNING: timer expired
    WARNING: timer expired
    WARNING: timer expired
    WARNING: timer expired
    WARNING: timer expired

You may get fewer lines here though.

The timer expired message should never be printed when the process(EXE).begin
probe is fired properly.

I tried the latest master branch of stap on kernel 5.0.16-100.fc28.x86_64 from
Fedora 28, kernel 4.15.0-72-generic x86_64 from Ubuntu 18.04, and kernels
3.10.0-957.27.2.el7.x86_64 and 3.10.0-1062.9.1.el7.x86_64 from CentOS 7, all
show the same problem. I just noted that this is much harder to reproduce on
kernel 5.0.16 than earlier versions of the kernel.

The stap version:

```
$ /opt/stap/bin/stap -V
Systemtap translator/driver (version 4.3/0.174/0.177, commit
release-4.2-10-g1427836ac118)
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: BPF LIBSQLITE3 NLS
```

Any hints on debugging this further? Thanks!

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